Off the top of my head, two main points for why Austin is better-suited than Houston for a tech HQ:
- UT (@Austin) means a large pool of fresh grads from a top-10 CS program they can recruit.
- Existing presence of both Amazon itself and other tech companies (Google, FB, MS, etc) means they don't have to relocate workers, and it's a lot easier to convince SDEs from other companies to jump ship.