Originally Posted by
pricej636 To keep the answer short and sweet, its because the carriers wont let them. The carriers dont want to and aren't able to support the random person installing a beta OS on their phone, and then calling saying their data isnt working, or their browser isn't showing up, or the dozens of other random questions that come up on this forums everytime a leaked OS hits. The bottom line is it would cost the carrier money.
Actually the even better answer is money

It costs RIM a fortune, a FORTUNE, every time a leak happens.
You would be shocked if I gave you some numbers on how much this costs, when RIM recieves a unit back from a carrier because a customer did something they shouldnt have, oh... I dont know, like install firmware they shouldnt even have, and then return the phone because its deemed "broken".