Winnipeg used to have a hockey team, the Jets, but in 1996 it moved south to the much larger city of Phoenix. Québec City also used to have the Nordiques, but they moved to Denver in 1995. Also in the mid-1990s, an expansion team was located in Atlanta, the Thrashers. But recently the Thrashers* re-located to Winnipeg (where they're once again calling themselves the Jets) and the Phoenix Coyotes are bankrupt despite getting some extremely generous subsidies from Glendale. This leads The Economist to conlcude that "the NHL's enthusiasm for the sunbelt is hard to fathom."
I think it's fathomable enough. The issue is the bifurcated cable television markets in North America. Right now, hockey has a national broadcast deal in Canada and it has another one in the United States. Losing the national deal in the USA would be very costly to the entire enterprise. But to maintain that national deal, the NHL has to present itself as offering a true nationwide sports league which means you need teams in the sunbelt. Atlanta's departure didn't leave too much of a hole in the map since there are teams in North Carolina and Tennessee. Probably any one of the remaining non-Dallas sunbelt teams is expendable, but if you lost more than one to colder climes you might start imperiling the League's claim to be a real major sports across the length and breadth of the United States.
* CORRECTION: I originally misstated the name of Atlanta's NHL franchise.