The purpose of a system is what it does. When some system has been around for a long enough time to have gone through several changes, and people have had time and opportunity to make changes, and nothing has changed, the conclusion is that what the system is doing is what is desired. If the people who could make the changes wanted changes, they would make them.
So, if the stated purpose is different from the observed behavior, the stated purpose is not correct.
The first presidential election state primary was held in Florida in 1901.
The purpose of the primary (and variations like a caucus, etc.) in the US political system was at one time probably to pick the candidate. Or to contribute to it by letting various voters see and meet and hear in person the candidate. Or representative of the candidate. They were spread over the calendar to allow for time to travel. In previous eras that meant by horse, then train.
However, much of that has fallen away. The purpose of the primary process today is to keep the candidates in the public view for months, even years. Various persons and groups have a huge interest in lengthening the political process.
- News organizations generate "eyes on copy" or clicks or sales or whatever.
- News organizations can exert their influence on the process by staging debates and playing intrigue games with how they are run, who is on them, what questions are asked, etc. and etc. This gives news personalities power they can cash in on after the election.
- Professional polling organizations get to take polls, and sell the results, for months or even years.
- Political candidates generate donations and endorsements.
- Candidates have opportunity to make more and more complicated overlapping deals and fall-backs and counter deals.
- Political parties generate donations.
- Meeting halls and arenas generate events that put bums on seats. As do hotels and restaurants and other "entertainment" venues.
- Various other groups get their moment in the public view, from unions to political action groups to protesters, to you name it.
- Party power brokers get months or years to make their deals, cement their power base, grease the wheels that need greasing, get on TV talk shows, slag off other people for being on TV talk shows, etc. and etc.
The candidate is chosen at the convention. It would be perfectly possible to compress most of this circus into four or five days. Party organizations could select their reps to the convention in their own way, out of the brightest part of the spotlight. This is how many countries do things, such as Canada. But that would, you see, defeat the purpose of this process.
So you won't get multiple states passing laws they must be first. It would defeat the purpose. The purpose of Iowa being first isn't for Iowa to go first, but to mark the start of the season. Nobody else wants to go first, they simply want to go at a time to fill a hole so the attention is never zero. Their purposes in organizing things this way are, at most, very weakly connected to picking a candidate as such.