Because it's awash in oil money and big companies who want interchangeable cogs for their .NET machines.

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.

It depends on what you're asking. Apple, Google etc. know that their employees probably prefer Austin so that's why they're there. A lot of software firms servicing Energy companies are based in Austin since its so close anyways e.g. https://mineralsoft.com/.