Austin is a very popular "2nd Campus" for tech companies, but it's not a great "HQ".
1) Poor airport connectivity 2) Seriously bad traffic options, with limited ability for build out 3) Not a lot of great public transit options 4) Limited ability for free land/property tax abatements/government handouts
Austin is a strong candidate because of UT Austin, but UT Austin students primarily come from Austin or Dallas (not like they are trapped on an island) unwilling to move out.