That's because a minute of usage for both Skype or long distance calling is easily quantifiable. If I talk on the phone for 10 minutes at a long distance rate of 10 cents a minute, easy, that's a buck.
Heck, it was easy 10 years ago when we had WAP phones that used circuit switched data, because a minute of browsing the web meant a minute of airtime.
But how do you tell an unsophisticated user that given the bit rate and duration of a video on Youtube or song on Pandora, he'll use X amount of data? What is the best way to explain how many web pages or Google maps driving directions they can access before he bumps into the limit? How much data is behind a Foursquare checkin?
Edit: ATT puts per-session data usage on detailed bills (meaning my data bill would be 30 pages long if printed), but its altogether useless at trying to match back to what you were doing when you generated those data usage records.
On the flip side, even as a "sophisticated" user, I can't tell how much bandwidth a given video will use, or how long I can afford to stream Pandora to my phone. But it doesn't mater. My carrier provides an app that I can use to check my usage, and I can see how much I've used as a percentage of my limit. It's easy to get a sense of "been watching a lot of video lately, gotta ease up on that," or, "oops, downloading that ISO over tethering wasn't a good idea." It's not long before you have an intuitive sense of what things cost, in terms of bandwidth. I find I don't have to pay much attention unless I use tethering, but my data plan has 6GB cap.