The chief reason is because patient autonomy is considered a key value in modern medical ethics, and that forcing people to get vaccinated violates this. Patient autonomy states that a patient should be able to make their own decisions relating to health care given to them (and, by extension, make decisions health care providers disagree with).
Patient autonomy is not an absolute rule. For example it is generally agreed upon that it's acceptable to violate patient autonomy in cases where the patient is not considered to be sound of mind and unfit to make rational decisions (think mental illnesses).
When exactly patient autonomy should be overruled and to which degree is a debate that has been going on for centuries, but it generally doesn't apply to cases where a person is of sound mind and makes a decision which violates the best available scientific evidence (or more crudely put, is being stupid).
A second reason is that modern government usually don't take a stance on "truth" unless there's a pressing need to. There are some philosophical reasons for this; how do we know who is right? In other words, what is truth? Can we know what is true? How do we know something is false?
These questions are not easy. There are real limits to what we can know, and some things can be considered unknowable. Specifically, I can't prove that vaccines don't cause autism. I can only show there is no credible evidence for that claim discovered thus far, but that's not the same as disproving the claim. This James Randi lecture explains the concept in some more depth.
Some might say that this distinction is somewhat philosophical, and that in practice we can prove that vaccines don't cause autism (this is what Randi argues), and this might be true for matters of science, but in this context it's not a scientific question, but rather a question on ethics and how to organize society.
The current prevalent attitude is that personal beliefs – no matter how irrational or contrary to evidence – are "sacred", and not something that the government (or society) should directly interfere with.
This is why we have free speech, freedom of religion, and all the rest of it. Much of this is just a rephrasing of "freedom to believe in your own reality". And "freedom to believe" is worth very little without the freedom to actually act on those beliefs. Which, in this case, means not vaccinating.
Arguments in favour of mandatory vaccinations
None of the above is absolute though – very little is – and there are some arguments in favour of mandatory vaccination programs, which I'll briefly outline below.
Other people may be disadvantaged by a patient's decision
The foundational rule of medical ethics is do no harm (Hippocratic oath). But do no harm to whom, exactly? A single person? Or a population? Vaccines are typically not especially beneficial to a single individual, but are to the entire population due to herd immunity.
Some people view vaccinations so beneficial to the population as a whole that it may override autonomy in some cases. There is some historical precedent for this. Opposition to vaccines is not new, and during the 18th and 19th centuries a number of countries made the smallpox vaccine mandatory. For example in 1902 the United States Supreme Court declared that a state has the right to order vaccinations to protect the people from a devastating disease, while also recognizing that individual liberty was important (Jacobson v. Massachusetts).
Since then a few things have changed though. Medical ethics standards have become more codified, and individualism has significantly gained influence in Western society, so autonomy is typically given much greater weight. In addition, the medical crisis of the 18th, and 19th century were considerably more acute. The measles outbreak you cite is terrible, but on the other hand it has killed "only" two children, whereas the smallpox outbreaks of the 18th and 19th century typically killed hundreds or more, and presumably would have killed many more if there had been no compulsory vaccination.
All of this is part of a larger discussion when exactly the general good will override the individual rights. There are no clear answers here, and there probably never will be. It's a personal value judgement, and if you ask 100 people you will likely get 100 subtly different answers.
However, we must consider that the effects of not vaccinating children are comparatively mild in "the grand scheme of things". The risk of actually getting ill is very small, and the total number of people who have gotten ill (or even died) is very small as well. Especially if we compare this to other preventable diseases such as smoking or obesity.
The affected are typically children
Relating to patient autonomy mentioned earlier is informed consent, which is exactly what the name suggests it is: a patient must be informed of the health care action, and she must give consent before the care giver can perform the health care action.
Children are usually considered to lack the decision making ability to give informed consent for medical procedures. It's acceptable for a care-giver to give an four-year old an injection even though it's screaming "I don't want to, it's going to hurt!" as the child cannot be informed and cannot give consent, so the child's parents (or legal guardians) have to give informed consent by proxy.
It's generally accepted that a third party (usually the government in some form) should intervene if the parent's decision or actions counter the child's best interests in such a way that the child is seriously harmed.
But where do we draw the line? Intervening on every possible thing that might cause harm is impractical and undesirable (it would be a absolute dictatorship) while never intervening would lead to abject abuse (paedophilia, child labour, etc.)
This, like the previous section, is a personal value judgement. In general however, it is considered that not vaccinating a child doesn't harm the child enough to overrule this. As mentioned in the previous section, chances are that the unvaccinated will live their lives happily without getting measles or any other vaccine-preventable disease.
Besides, what's next? Will parents of obese children get in trouble? That's probably a lot more of a pressing problem than vaccinations.
Opposition to vaccination is not new, but it has seen a large surge in the last twenty years after Andrew Wakefield's (now retracted) 1998 paper published in The Lancet where he fraudulently claimed that vaccines cause autism. Even so, the practical effects have been fairly limited. Yes, there have been outbreaks, and yes, people have died, but the scope of it has been small and most children still get vacinated. So the pressure to actually do something has been fairly limited.
This may change in the future, though. As the effects of non-vaccinated people may become greater. And some countries (such as Slovenia) do have mandatory vaccination programs, while some other countries (like many U.S. states) have some limited mandatory vaccination program with various opt-out clauses. Some politicians have also offered their support for stricter mandatory vaccination programs, such as Hillary Clinton and Ben Carson during the Presidential campaign.
It's also possible that a new lethal disease will rear its ugly head in the future, like the Black Death did in the middle ages. Should such an epidemic arise, vaccines almost certainly will be made mandatory, but this is not what we're talking about here.
This answer comes with some caveats:
- There are a lot of subtleties and exception that are not covered. You can write a book in response to your question, so I've had to choose between brevity and, well, writing a book :-)
- I've generalized a lot, which is why I used the words generally and typically so often. There are a lot of differences and subtleties in attitudes to this topic, but I think – hope – I've captured the essence of it.
- This applies to Western culture. I have no idea what the prevalent attitudes are in China or India. You mentioned Romania in your question, I also don't know anything about the on-going debate there, but in general, these are the most important considerations in the debate.
Some interesting links, some of which were used as a source for this answer.