Format changes are less common for a few reasons...
1. Most medium to small markets have only a few big owners. They have the money to do the research and have likely got all the bases covered. If iHeart or Cumulus has been in your market for a decade or two (by previous names), they know if your market needs multiple Country stations or several Rock stations. No surprises.
2. Most markets have seen a few FM signals taken out of competition. Maybe EMF, VCY or another religious broadcaster has picked off a weak competitor and converted it to non-commercial, listener-supported Christian Contemporary music or Christian Talk and Teaching. In Cincinnati, EMF bought 104.3 and made it K-Love. It also bought 96.9 in Dayton and made it Air 1.
3. A public radio operation buys an underperforming signal in your market and converts it to non-commercial AAA or Classical music. In nearby Columbus, Ohio State University bought 101.1 and made it Classical so its main signal, 89.7 WOSU-FM, can be all news and information.
4. Maybe your market has seen its Spanish-speaking population grow and one or two of your FM signals are now Spanish-language. Again, the big owners of Spanish language radio do their research. They now how many people in your zip codes are primarily Spanish-speaking. Once they switch, they're not likely going back.
5. And the sad truth. Radio is less dynamic than it once was because fewer young people listen. You can't just take a low-rated Easy Listening station like WVNJ-FM in North Jersey, start a new Top 40 format and turn it into Z100 WHTZ New York, going "from worst to first." If you flip a so-so station to another format, it won't make as much of a splash, no matter how good the effort is. People won't be talking about it because fewer people listen to broadcast radio. Those who do listen to radio are older and less likely to follow the latest trend.