I don't actually follow the selection process closely enough to be able to argue whether a San Francisco belongs in over an Indiana. However, I think putting the mid-majors in makes the tournament far more interesting (at least to me).
I'd far prefer a 6 vs 11 matchup with a #11 San Francisco/Wofford/Lipscomb trying to knock off a power conference #6 seed, compared to a #11 Indiana/Nebraska/Florida trying to do the same. I understand that fans of Indiana/Nebraska/Florida feel differently, and that those names draw higher viewership than some team that only people on this message board have heard of.
That's a big part of it for me, too. We already have enough Power Conference teams in already. I cringe when I see teams get in with 13-14 losses - and teams like Saint Mary's are left out. Last year I want to say SMC went 3-2 verse the top 50 (low sample size, admittedly), and they were in the top 40 in every efficiency rating. They also went 29-5 in the season.