Understanding Texas A&M's decision is easy. Courtesy of Chip Brown at Orangebloods.com:

...sources say UT officials have vowed that if A&M turns down its invitation to the Pac-10, breaks away from UT and goes to the SEC, Texas won't schedule the Aggies in anything anytime soon.

Texas A&M's athletic identity revolves around hating Texas. Ending the rivalry with Texas would likely be detrimental to Texas A&M in the long term as losing its only important rivalry would weaken fan support. To put it another way, Texas A&M losing its rivalry with Texas would be like Michigan State losing its rivalry with Michigan, only worse.

Meanwhile, Texas has two compelling reasons to stay in the Big 12. Firstly, by staying in the Big 12, they maintain maximal flexibility as they can go to any conference of their choice while having a huge amount of power over the Big 12. Secondly, they can form the Longhorn Sports Network, which would not be an option in the Big Ten or Pac 10. And if the network doesn't result in a major financial windfall, the Big Ten and Pac 10 are both fall back options.