I'm not terribly knowledgeable in this area, but I think part of it has to do with the availability of frequencies to do data communication over. They price data so people curb their data usage thus preventing crowding on the already limited bandwidth.
Except that Sprint and T-Mobile do offer unlimited data plans (without throttling) and T-Mobile simply throttles instead of charging overages for its non-unlimited plans. I've used both and have yet to encounter issues with congestion (though I don't live in a big city, so maybe it's different in that environment).
Non-unlimited plans with throttling instead of overage charges would probably be the best solution to overcrowding issues in the short-term.
Of course, this could just be FUD I've been fed.