First, here's the story in case you want to read about it. Now let me talk about why this happened (again) to Atlanta.
1) Miserable ownership. The Atlanta Spirit Group (ASG) have been a bumbling mess from the start. They didn't have a hockey guy amongst them and it showed. They did little to market the team and when the economy crashed, most of their business deals shriveled up and died. I've seen some bad ownership (Cincy Bengals, Oakland Raiders, Pitt Pirates, LA Clippers, etc), but the former owners of the Thrashers were the worst of the worst. They traded away all of their stars (Heatley, Kovalchuk, Hossa) and got almost nothing in return.
2) Fan support. Re-read point #1 and you have a hard time blaming the remaining fans to spend their hard-earned money on slop. Still, ranking 28th of 30 in attendance didn't help things.
3) Location. Most of the fan base live north of the city. The Thrashers played in 'beautiful' downtown. Hard to get them to drive 60-90 minutes in traffic to see the slop on the ice. If the team played in Gwinnett Arena I suspect turnout would've been much better. The inhabitants of downtown Atlanta had no interest in hockey.
I used to go to 4 games a year. I stopped going a few years ago as the product on the ice was dreadful. Most of the people in attendance were there to see the visiting team anyways. I'll miss seeing live NHL games down the road from me... hate it that my kids won't get a chance to see it (unless I take them somewhere like Nashville or Tampa), but the hard truth is the NHL south of the Mason-Dixon line just doesn't work.
If only Winnipeg took the Hawks too... then I'd probably celebrate. As it stands there is some sadness in seeing the Thrashers go, but Winnipeg will support them (despite being 1/7th the population size) much better.