Lobbyists and image. Both sides of the LPFM debate have powerful lobbyists (NAB is of course more powerful) but the image of helping out the "small town" guy over the "700 station corporate behemoth" is also a powerful motivator on both sides of the aisle.
Not to be nasty to my colleagues at the NAB, but they, too, are "small potatoes" to the FCC these days. Looking at the NAB's track record, they haven't been too successful in their lobbying efforts in the last ten years. I would opine that a major reason for that is because while the NAB is a lot bigger than, say, Prometheus Radio (LPFM), both are small change compared to Verizon Wireless, AT&T, T-Mobile and even Sprint. Whoooooole lot more money involved with wireless services (i.e. internet over cellphone and wifi/wimax) than can possibly be involved with one-way broadcasting like radio/TV. And that's before broadcasting had all its "financial issues" of the last four or five years.
In a sense, this has led to a great leveling as far as Congress is concerned. Sure the NAB can drop a C-note when the LPFM crowd can barely scrape together a few pennies for lobbying...but wireless services are like a Scrooge McDuck Money Vault
in this analogy. THAT'S what Congress and the FCC really care about. Viewed in that light, it's better PR to throw some bones at the LPFM crowd than to chase a few extra dollars from the NAB.
Granted, this is armchair analysis at best, but I don't think I'm wrong. Other opinions are welcome.