How much for the data only plan?
Scratch that, I know perfectly why. First is money, they can make a whole lot more by charging you for each text message and each minute. Second is technology, phones only recently have become fast enough to do the proccessing for VoIP. Wireless technology is just starting to catch up in the US to the point where you have enough bandwidth to make VoIP calls without resorting to WiFi.
We've seen it happen with landlines so it's only inevitable that it will happen with wireless phones. It's not gonna happen anytime soon, I predict it happening in ten years at the earliest but I would put my money on it.