The problem is that nobody watches. People talk about loving Cinderella but nobody watches those games until the last minutes if it’s close and despite those upsets being memorable moments, they are NOT profitable. Low ranked schools lead to low viewership. When you had FAU, UConn, and San Diego State in the final four, nobody watched and it was the lowest rated final four in many years.
Having a only P5 teams would increase the total watch time for the early round games and would purposefully exclude the lesser brands from their bigger games that attract bigger audiences.
Writers and casual fans love to romanticize Cinderella, but most real fans want to watch Kansas, Kentucky, UNC, Duke, UCLA, Indiana, Ohio St, Florida, LSU, Arizona, Texas, and so on…in the S16/E8/F4 and that is what sells ads which is what drives the market rate for the entire tournament.
It’s not a popular idea, but it’s factual. The P5 schools don’t want to be upset/embarrassed by the Ivy League or MAAC teams, and to rub salt in the wound, most of the times when that happens it tanks the ratings as well which takes even more money out of the pocket of the P5.
While the loss of big brackets and Cinderella will likely hurt in the short terms, but in the long term it would be more profitable.
You can argue that this shouldn’t be about profitability, but then you likely haven’t been paying attention to the last 30 years.