Just a hunch, like
@Tgar has hunches, that the conferences and schools said: "For the opening games we don't want lesser/middling/piddling bowl games that suck all the revenue from the games off the backs of our programs and players. We want them on campus and the CFP/Conferences/Schools control-get all the revenue."
And the 4/5 major bowls probably also don't want an opening round showcase at lesser bowls, thus raising their status.
And on-campus hosting removes any potential scheduling conflicts with potential NFL stadium sites of those lesser bowls.