Why did the Mastodon become extinct?

There is growing evidence that a comet strike is responsible for the extinction of most of the large mammals including mastodons and humans (the Clovis culture) in NA about 13,000 years ago. Discover magazine has an article in the April 2009 issue. Their extinction has also been linked with their yearning to go down for so long that they simply stop breathing. National Geographic has an article in the June 2008 issue.