Men’s Water Polo Just guessing, but relatively minor sport, limited geographical participation, historically the sport has been dominated by the four major California universities. For a long time the entire tournament was just four teams; the winner of the MPSF (think Pac-12 and at one time Big West), the winner of the WWPA, a team from the east coast, and an at-large which was typically the runner up in the MPSF. Several teams (mostly Big West and West Coast Conference schools) broke away from the MPSF, I can only assume because of the domination by the big four Pac-12 schools, and created the Golden Coast Conference. They now get an auto-bid. The very first "Opening Round" or "Play-in" game is now on the Saturday following Thanksgiving and features two east coast teams.
Several small schools left the WWPA a few years ago and play in the Southern California Intercollegiate Athletic Conference. Those are the same small schools, mostly the Clarement, Whittier, Mudd, Pomona-Pitzer schools that play in that conference for other sports. As of 2019 they no longer participate in the NCAA tournament. Men's water polo, like men's volleyball, only has the one tournament, theoretically open to all divisions. There is no seperate NCAA sanctioned tournament for what are otherwise D-II and D-III schools. A seperate organization, US Water Polo, is sanctioning a tournament for NCAA Div. III level schools for the first time this year. There are only a handful of D-II schools that play water polo.
But then, this is all a guess on my part.