Because Iowa caucus goers are serious and take the process seriously. If you haven't participated here you have no idea. The candidates visit communities and people actually travel to participate in those events. The candidates realize they have to connect, and they work to do it, My first event was last summer. And events pop up at any time, even in off years.
Iowans realize the importance of, and the impermanence of their position of 1st in the nation. To this end the caucus goers are well informed and self-educated. The caucus system is complicated. The problem creates a virtue; an infrastructure of personal connection and communication that remains.
There is something to be said about diversity. But the loss of personal interaction offsets that. If Iowa loses its place as 1st in the nation but keeps it Caucus process, I don't see how the Candidates can avoid coming back to Iowa,