The almost-too-painfully-obvious-to-even-say-aloud answer is that AT&T will make more money by not offering rollover data, so they aren't. I don't expect this will change anytime soon. AT&T has done nothing to suggest they have any interest in providing value to their iPhone and iPad customers, but let's imagine a different world, one where AT&T hoped to keep some of their iPhone customers, perhaps a world where other carriers had the iPhone and AT&T had to actually compete for their business.
Read on to imagine with me...
I've taken AT&T's description of rollover minutes and applied it to data:
AT&T proudly offers Rollover® Data. That means if you don't use as much one month, AT&T lets you keep your unused megabytes for next month.* Rollover Data is unused, accumulated data that carries over from month to month for up to 12 billing periods. The benefits to you, our customer, are that you can keep unused Anytime Data and use it when it's convenient for you. You can avoid paying Additional Data charges by using your accumulated Rollover Data instead. Rollover Data is handy for those times in your life when you need more data.
Consider my wife's iPhone data usage: when I looked back over the past seven months (which was as much as I could see), the most she had used was about 700MB. In four of those seven months, she had used less than 200MB; in three of those seven months, she had used more. All AT&T offers is either 200MB or 2GB. One is too little, and the other is too much. I'm sure that others are in a similar situation. Imagine if I could spend $25 for 2GB of data usage for my wife, knowing that she could use it whenever she had need, but it wouldn't expire at the end of the month.
Almost everything is already in place to make this happen: AT&T is used to monitoring data usage, and the iOS will report when you are nearing your data limits and offer to let you renew. The iPad is already setup to let me choose how much data I think I'm going to need... but what if I'm wrong? I've had my iPad data plan in place for a little over a week and I tried to "guesstimate" how much data I had been using, only to find out I was really wrong. How many people can successfully estimate how much data they use or will need? I bet far fewer than can estimate their voice minutes needs.
For one thing, "minutes" are much easier to understand than "megabytes." I know that when my mom calls, I can expect the conversation to be at least 10 minutes. But how about those emails that she forwards, the one with all the "funny" pictures in them. How much data did that cost me?
The only thing that's left is for AT&T to figure out how to deal with rolling expiration, but they've already done that with voice, so I wonder how difficult can it be for data. If there's a will, there's a way.
Sadly, I expect that no one at AT&T has the will, so there's no way -- their current plans are probably more successful than ever with the iPhone 4, so there's no reason for them to change over to a plan like this. But maybe we can hope.