It because that's mainstream. One of the biggest points of punk rock is to go against conformity. They want to stay underground. If you are a true punk rocker...you'll find them. Once it hits the radio it's considered, pop punk and is looked down on by most of the punk rock community.
I think that most radio stations have a certain theme or they have a schedule for the type of songs to be played. This could be the reason why you do not hear more punk rock an alternative songs on the radio. Another reason could be that these songs do not have a lot of audience.