Commercial radio stations support themselves by selling advertising. The advertising dollars are based on the station's ratings. Ratings depend solely on how many people listen, how often, and if they accurately report their listening habits to Arbitron, the company that measures radio listening.
In addition, advertising agencies and businesses buy commercials based on demographics. Some demographics are more attractive than others. Sometimes, stations change because winning high listenership in a particular demmographic would be more profitable.
When ratings fall or the current demographics become unattractive for future sales, it becomes harder for the station to sell ads or "spots" as it's called in the business. A station can only hang on to a format for so long before it seizes an opportunity to change in the hopes of higher ratings.
To the devoted listeners of a particular radio station, a drastic change is usually viewed as an atrocious act of betrayal. But, to a radio station, a drastic change is simply a business survival move to stop losing money or to make more of it.
Trust me: if more people were listening to the station or the demographics were more attractive, it would be less inclined to change format.
Commercial radio stations don't care what format they are, as long as it's profitable.