Because Phoenix is 12th in TV market rankings at 1.89 million households and San Antonio is 31st with 939k.
By the way, Tampa is 11th and Detroit is 13th. I go back and forth between whether a Miami admission hurts Tampa's bid or not, but it is undeniably the largest untapped TV market, and larger than both Orlando and Miami. If we let TV market size be our guide:
24 - Miami (16th)
25 - Sacramento (20th)
26 - St. Louis (21st)
27 - San Diego (28th)
28 - Tampa (11th)
29 - Phoenix (12th)
30 - Detroit (13th)
Cleveland, Charlotte, Raleigh, Baltimore, and Indy are the only markets bigger than San Diego that wouldn't be in yet, but only by 400k in Cleveland's case and 200k or less for all the others. Nashville is right behind at 29th, and Cincinnati's at 36th.
https://en.wikipedia.org/wiki/List_of_United_States_television_markets
It's good to be a little careful with these rankings. I was surprised to see that Miami is ranked 5 spots below Tampa, even though Miami is a much larger city. It turns out, they cast a wide net for the Tampa market, including Sarasota/Bradenton, for example. But they don't do that for Miami - excluding West Palm. If you add the West Palm/Ft. Pierce market (#33), the combined total rises to #6.