It does not really have anything to do with the amount of data and networks, that is just smoke and mirrors. They want an excuse to make money.
Well it started out as a tiered plan (early 2000's), but once someone started to offer unlimited data, the rest where forced to. After att, and exclusivity of iphone started going tier, it was easy for verizon to lock a few good phones in and go tier.
It is about money. Att and verizon are now facing each other. The best way to stop people leaving is make sure the door hits them on the way out. Att is not going to go back to tiered so verizon never has to.
If you leave your current provider, you are screwed. They are doing it for three reasons. 1) They can, there are only two national providers now. 2.) It locks current subscribers in and gives them zero reason to leave, no one in their right mind would give up unlimited for anything. 3.) It makes them an insane amount of money.
With the phones locked to the carriers and the carriers only having one person to fight off, it is easiest to hold your unlimited plan hostage.
Sprint is dead in the water for the foreseeable about future.