No one’s disputing that Atlanta and Houston are bigger markets. The NHL has already tried and failed to make hockey work in Atlanta twice. Houston, I’m sure, will be an expansion location eventually.
The Flames were put in Atlanta simply to keep the WHA out of there, and the eventual Thrashers ownership literally did not want the team to play in the building they owned. Atlanta is not a good pro sports city, but the Hawks and Falcons are still there, despite all their incompetence over the decades.
Put a decent ownership in Atlanta, and there's no reason the market shouldn't work. At least to have a team there. That's the problem the NHL has had though. A lot of ownerships have been awful, and approved for short term interests. Even Arizona, or Phoenix, was the lesser choice, since originally, the Jets were supposed to move to Minnesota to replace the North Stars. Then Phoenix eventually became a whole thing on its own. But, even in a hockey hotbed like Minnesota, the North Stars were in financial trouble back in the day, and had to merge with another failed franchise in the Barons to remain intact.
Basically, everything since the doubling of the league in 1967 has been a bit of a mess. Even the Blues, who made the Final in 3 straight years after expansion, were pretty much a favor given to the Hawks owner.