Well, for me it's because ATT data speed is at least 4-5 times as fast as VZW.
Surely you are kidding? 4-5 times faster than Verizon?
Have you used an ipad with both carriers?
I have used AT&T on an ipad 1 since April 2010 and since last Friday I have used it alongside an Ipad2 on Verizon. There has been rarely a case or scenario where AT&T is faster than VZW. On a speed test, AT&T has higher ping times than VZW. Most of the time the data rate is the same as VZW, sometimes it is faster, but in actual use (browsing, loading images etc) the VZW has been approximately 20-40% faster virtually 90% of the time.
This first video is using the Speedtest.net app and shows ping times and download/upload speeds.
http://www.youtube.com/watch?v=ai4wswF6FU0
Both Ipads are using the built in Cellular card, no Mifi.
The AT&T is an Ipad 1 and the Verizon is an ipad 2. I will have an ipad 2 for the AT&T in a few days and will rerun the test.
This is not an issue of Verizon being 3G and AT&T being on Edge, both were 3G with 4 or more bars of service.
The consistently high ping times on AT&T are causing latency which causes lag. In all of my browsing the Verizon feels and act much faster than AT&T.
This video is using DSL Reports mobile page, which does a test by downloading a file multiple times and then calculating an aggregate throughput. I also downloaded a 2mb image to gauge actual page load times.
http://www.youtube.com/watch?v=6WH_NzHFM2Q
Based on my testing, albeit somewhat unscientific, Verizon performs better in my area. That may not be the case for all areas. Clearly if either provider doesn't have good coverage in your area, they will not be a good choice.