you answered your own question.
greed
The potential profit from tiered data and overages outweighs the gain in marketshare. They have nothing else to charge us overages with, unlimited SMS unlimited Minutes... no unlimited data. until there's a new feature they can charge us for on our mobile phones data is the gold mine.
They can charge late fees.. disconnect and ETF... but compared to data overages and tiered data plans the amount is absolutely minuscule
How so? If they increased their market share by even 5%, that is massive. That influx of millions of new subscribers, would offset the small hit per person that they would take on an Unlimited plan, knowing that the average person won't use any more data than they are now...
The truth is that not that many people CARE about Unlimited plans, so there is no real reason for VZW to keep offering them. Wifi is at home and in so many places, and a major segment of the population spends time on email and low bandwidth stuff...