It’s farther than you think. Nearly twice the distance from Philly to NYC
Not only that, but a lot of folks in the greater Austin-San Antonio area can't even see Astros games.
Spectrum (formerly known as Time Warner, now owned by Charter) is a very prominent cable provider down here, and they don't carry the Trashtros' home network since it's owned by AT&T (Spectrum's major competitor in most of Texas).
Putting a team in Austin would do very little to encroach on the Astros' fanbase.
Granted, I don't see Austin as a particularly good destination for a sports franchise. It's home to too many white collar types with marginal interest in sports.