There's a major difference between football and the other sports. It's physically difficult to play football more than once a week, while in basketball or volleyball or baseball or pretty much any other sport, it's a given you'll play at least two or three times in a week.
In basketball, volleyball, and baseball/softball, you can start a state tournament with every school in the state - because you'll play at least two rounds that first weekend and reduce the field by 75%. You can't do safely do that with football. It's the same reason why the people calling for a 64-team playoff in college football are wrong ("they do with March Madness, they need to do it with football, too!") - March Madness takes four days to go from 64 teams to 16 - there's no way you could do the same with football.
Now, I wouldn't be opposed to expanding the high school playoff field myself (I do think 24 would be a good number), but it's just not feasible logistically to compare the state tournaments in basketball or baseball to the football playoffs.The IHSAA has always been steadfastly against byes in the football playoff system (I don't know why, but somebody in Boone really, really hates the idea) so that makes 24 a tough deal to get approved.
And remember, we're up against the calendar, too. UNI will not allow use of the Dome during Thanksgiving week, so either you start with championships the week before (as we do now) or plan to play them outdoors somewhere. If you backtrack 13 weeks from the week before Thanksgiving, you come up with a late August start for football. Adding another round of playoffs means 1) starting even earlier in August; 2) pushing the championships into Thanksgiving week, which means outdoors; or 3) either compressing the playoffs as was done prior to 2015; or changing the 9-game regular season schedule.
