Some cities are good candidates for being called "capital" of a geographic region while others can be important places but not have that kind of relationship with their surroundings?
Atlanta has been such an important hotbed of culture and politics and economic activity in the South that you could call it the capital of the south.
To me, the things that make Houston important or famous are sort of stand-alone in nature, like the energy industry, medicine, nasa, tech, etc. I think being in proximity to Austin and Dallas diminishes how Texan we can claim to be while being in proximity to New Orleans means we can't really claim to be Southern/Gulf Coast either.
Sport teams allegiance could be a proxy:
Not saying it isn't just because the Texans are a younger expansion franchise but still...
source: Gizmodo, "A County-By-County Map Of NFL Allegiance"