>>52944187>due to the high number of "participants"Not really, otherwise Mario Kart and other events would get a similar number
There is a series of reasons, including the fact it overlaps the real Koshien and rides their mindchair wave.
Niji Koshien is just the equivalent of what could exist in an alternate timeline where HoloEN followed North American traditions and had enough girls to make a "March Madness" event with one girl adopting each team and ducking it out against each other in some simulated college basketball game
