Short answer: Because the sanctions were not the main cause of the most important current economic problems in the first place.
Both answers posted so far present good points, but I believe they miss the key factor here by accepting the faulty assumption of the question, namely, that the sanctions were the main cause of the economic hardships that have contributed to the current protests in Iran.
Strictly speaking, this is not correct. The oil sanctions that were imposed around 2009-2010 and lifted after the nuclear agreement, although affected the economy, were not the main cause of the current economic problems in Iran that is fueling the protests. This becomes clear by looking at the key economic indicators such as inflation, unemployment, and growth during the past few years and compare them with the past 4 decades. These stats show that inflation is now at its lowest after revolution and growth rate is going to be positive, around %5 (not considering oil exports), for the 4th year in a row. But unemployment is on the rise, which most believe is the main economic contributing factor to recent protests.
Here I explained why the unemployment has risen in the recent years (and still rising), and why it does not have much to do with the sanctions.1
Iran witnessed a huge population growth between 1980-1990s (from ~40m to ~60m, a ~%50 increase). It became concurrent with widespread availability and access to education (especially college education) throughout the country for both men, and women. This part of the population (lots of them being women) was to enter the job market after graduation from college, expected to be around 2006-2020. The enormous increase in the demand for jobs by this young, educated population could be (and was) predicted by some economists since late 1990s. The following is from an economics book (title: Industrial Development Strategies) published in 1999, in Persian:
In the coming years, the job market will face a huge number of young, educated people seeking jobs, lots of them being women. Our economy should either get prepared for widespread poverty caused by ubiquitous unemployment and low paying jobs, or become able to create over 1.1m to 1.2m jobs per year by performing fundamental changes in the economic policies and priorities.
This is what happened in reality (created based on the stats from Iran Statistics Center):
(Red shows Ahmadinejad presidency years, and green shows Rouhani's.)
You can see that even before the oil sanctions, the government's performance has been very poor with almost 200k jobs created per year. The end result was widespread unemployment.
(It is worth noting that oil prices substantially increased around 2008, creating a unique opportunity that could give the government the means to catch up with the population entering the job market, but unfortunately it is still not clear where all that money was spent on. This occurred during the presidency of Ahmadinejad, who is seen by almost all experts to have had extremely poor economic performance. Then came the sanctions, reducing the oil revenues close to zero and making everything worse.)
This is why unemployment is becoming prevalent especially among the youth, and is expected to rise in the few next years too. Therefore economic mismanagement by the governments especially during the past 15 years, was the main cause of the current economic situation. What is stated in @senaps answer is just a part of this mismanagement, and the point raised by @FooBar is correct that some sanctions although lifted on paper, are still in effect in practice.
The current protests are not fueled just by economic hardships. The protesters have political motivations too, such as limited personal freedoms, women's rights, corruption mostly caused by intransparency and unaccountability of the military forces, specifically IRGC who is unconstitutionally a big player in the economy, etc.
In conclusion, the sanctions (and the relief from them) does not have much to do with the current protests in Iran. They were not the main cause of the current economic problems in the first place. There are much deeper roots and more long-term reasons for that.
 This is based on a recent talk by Masoud Nili who is a professor of economics at Sharif University, has been a member of many key economic councils in the government in the past 3 decades and is currently the economic advisor of the President.