So Why Is Iowa the First Primary Event?
Here's my take on that:
By choosing a small, low-population state as the location of the first primary event, candidates who don't have a big campaign fund can have a chance to make themselves known. Iowa, as a caucus state, provides lots of opportunities for candidates to hold small meetings of voters in the many precincts in the state. It also is easier in a small state to assemble a campaign team to do GOTV efforts.
Iowa is also a mixed state, politically, that is reasonably equally divided in voter preferences. Its racial demographics are off the norm, but that doesn't really play into it that much. It also has a mix of urban and rural voters, something people want to test in the primaries. It has a decently representative age spread and educational and income demographics.
If Iowa were replaced as the first primary event location, another small state with a small national convention delegation would be chosen to replace it. One state. The reason is to allow candidates to throw their hat in the ring without having to spend huge amounts of money in a state that has little effect in the convention nominating process.
Why not do a Super Tuesday sort of even as the primary kickoff? Same reason. It would be too expensive for many candidates, and thus less available to them. So, we have one small state as the first event. Do well in it, and fundraising becomes easier, so the poorly-funded candidate at least has a chance.