Good stuff here from Slate blogger Matt Yglesias:
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.
Offered in response to The Economist.
Update: In case you missed it, more of my thoughts on the future of national television broadcasts for the NHL from March 30.