Defying Gravity is around 6 minutes and the buildup to the climax (which only lasts for around 1 minute) making not the ideal peice.
As for Hamilton, there's no real song that would work unaltered for radio. My shot is the closest, but even then, it's length could be the reason why not everybody is listening.
But why you wouldn't hear Broadway on radio is the fact that people know what they want to hear. If you turn on your radio to a mainstream station, chances are that they are just cycling through 5 songs, with an equal amount of ads and content. The fact of the matter is that there's no room for Broadway. I mean, most artists that are mainstream can't get their songs on the air.
If you want to hear showtunes on radio, you probably have to go to siris or your local college station (look through their schedule.)
I appoligise for any spelling mistakes. I may be on my mobile. Clumsy fingers and small little touchscreen keys don't mixx. I try to spellcheck, but I may miss something.