People don't realize what they miss out on till they have a taste of it. Then they never want to go back. In example, I'm sure that there's a significant amount of the population who would be just fine with dial-up. But they got a taste of broadband. You think that after getting such, even at just 1.5MBPS, they'd be willing to go back to 56k?
The same logic is applied to data usage. You have no data cap, you grow accustomed to using significant amounts of data if it becomes available for you; and it's quickly becoming easier and easier to do so on mobile. Between games (and they're getting bigger and bigger for phones and tablets), video (whether it be YouTube, Hulu, HBO GO, Netflix, or remote video such as Vulkano (Hava), or Slingbox), remote desktop, etc., let's face it: Data usage is increasing.
Hell, Comcast (and I think, but though don't quote me on this, Time Warner offers it as well) advertises the ability to remotely watch programming on mobile devices.
Then there's the olympics and streaming that over phones...
Data caps, let's face it, are antiquated relics of a past generation of mobile providers charging .25 cents per megabyte.