Historically, a base has needed 20-ish direct flights to other cities as Step-1 before consideration for base. It has also needed on-site local Airbus maintenance talent, contractor or otherwise, and hangar space for overnight maintenance or maintenance while bad weather existed.
A large number of TDY/RON/overnights in that location was also a consideration, as this cost the company alot of money.
IAH has 6 cities served by Frontier
HOU has 3 cities.
CLE and CVG each have 11 or 12 cities
so...