Austin and San Antonio are close by, but there's a noticeable gap, and nearly no one lives in Austin and works in San Antonio or vice versa. Austin and San Antonio don't have the same housing prices, salaries, etc. Contrast that to a place like DFW where the boundaries all blur together and people cross-commute all over the region, and consequently they're lumped together in the census area.
To put this one last way, the census area described as Boston is about 60 minute driving radius around the central city. On the best of times you could make it from downtown Austin to downtown San Antonio in ~80 minutes, and in practice they each have distinct commuting sheds of 45-60 minutes that partially overlap around San Marcos.
Austin is great, and also it's half the population of Boston :)
It's also about institutional knowledge. It's (relatively) easy to find people in Houston who understand the value in spending $10mm to drill an oil well which has an 80%+ chance to make a single digit multiplier of your money. There's less cultural knowledge of spending that same amount of money for much smaller chance of a much, much bigger payout.