It's because people like what's familiar. Most radio listeners are probably not hardcore music fans. They're people that like to listen on their morning/afternoon drives, while they're in the kitchen or at work, etc. If they hear what's familiar on the radio, they stay on the station. If they stay on the station, tney hear all the commercials. And that makes advertisers happy. And the bottom line is this - say it with me now -
Radio is not about music. It's about money.
Of course, the question now is: if radio only plays the songs that everybody wants to hear, how does new music ever get added? That answer involves more detail than I can write here, but involves people being surveyed by phone, and it also involves cross-marketing promotion from the record companies...
In the 1960s and 1970s it was very much a different story - radio was more progressive and free-form, and you could play Miles Davis next to Jimi Hendrix next to Neil Diamond...but that's another story altogether.
The best way for you to hear good and new and different music on the radio is to either a) seek out a public or college radio station, b) subscribe to Sirius or XM Satellite Radio, or c) listen to internet streaming stations like
Radio Paradise. I currently record streams of Radio Paradise to mp3, and put them on my iPod to listen to in the car - so I can hear new and varied music without commercials.