FM channel allocations are limited commodities. When you get a construction permit to enable you to build a radio station, it is because you have proved to the FCC to its satisfaction that the signal that you transmit from that tower at that power at that location will not cause co-channel or adjacent channel interference to pre-existing FM stations. The process is a lot like making cookies with a cookie cutter and a roll of dough. You can make cookies as long as there is a big enough piece remaining to make a cookie.

When commodities are limited and valuable, competition for them is vicious. The big boys with the big bucks will spend those big bucks to keep the little guys from getting an allocation. Back in the early sixties, FM radio stations were easy to get and cheap. When the US switched their listening habits from AM to FM in the early seventies, the game changed.

If you want to read a technical explanation, read this article on FM Broadcasting in the United States. Pay particular attention to the information on FCC Docket 80-90.