a few points to make:
1. Some people make art for the sake of the art itself, not for the money. Some people have 9-5's, but still want to create something and put their stamp on a particular artform and are not motivated by money to do it. What's happening in hip hop is no different that what's happening in other genres of music. You'll be hard pressed to find classic jazz styles on the radio, but the airwaves have no shortage of "Smooth Jazz" stations that won't touch a John Coltrain, Art Blakey, Wynton Marsalis or the like. There's "dance music" stations that'll play progressive, trance, and "dance remixes" of popular songe, but won't play Detroit Techno, Chicago House, or anything like that. Often, artists that get no radio play, irregardless or genere, find success on the road, especially overseas where there's an appreciation for original, creative music. Jazz musicians, Underground MC's, and Techno/House DJ's all make theirs on the road.
2. Ever since the Telcom Act of 1996 passed, radio station ownership has gone from independent stations in every market to every station owned by either Clear Channel, Radio One, or Emmis Communications. Before the bill passed, a radio station's programming director made the bulk of the decisions as to what got played on the station and what didn't, with DJ's having the ability to break records they though would do well in that listening area. It wasn't uncommon for a local rapper to cut a 12" single and hand deliver it to the DJ's at a given station, chop it up with them for a bit, and minutes after they've walked out of the building have their single being played during prime time. Right now, this will almost NEVER happen. Station programming is usually decided by a regional manager and the playlist is sent to the DJ's to spin. Deviation from that playlist can often result in termination at many stations unless the show specifically calls for it and it's arranged and approved ahead of time (i.e. morning or afternoon commute mix segments by in-house DJ's).
3. To get inclusion in a regions playlist, record labels pay an Indie promoter to push a song to the regional directors and offer "gifts" to the stations under that individual (sometimes being able to work directly with the station itself). This has gotten labels in a ton of trouble in the last few years, but even with court rulings and fines, the practice known as "payola" continues as a way of doing business. Because "underground" rappers are usually on smaller labels, they can't get the same exposure because their label can't afford to pay an Indie to get spins, sometimes opting to go the college radio route.
There's a lot more to it, but in a nutshell this is why more gifted rappers aren't on the radio.