AT&T had the market and was the target. Now that everyone has the opportunity to bail on AT&T, they are not doing as expected. Sure, some will switch but it seems most wont. Would you switch to a significantly inferior network, to use only part of your iPhone's abilities, just so you can take a chance at 1 or 2 less dropped calls a week? I am in the group that understands that cell phone technology comes with dropped calls and your going to get them on Verizon also.
Apple is extremely careful about what technology they use. Many times they are considered late to the game because they choose not to embrace risk that could reflect negatively on their products as a whole (ie... no blu-ray on Mac Products). I believe Apple made some of their decision to initially offer the iPhone on AT&T because they considered it the best network and most likely the network best able to handle the functions of the iPhone.
AT&T did a exemplary job and partnering with Apple to change the smartphone market.
Again, I thank AT&T for taking the heat and providing a fantastic network that exploits the iPhone.