Because I don't see too many people tuning in to an hour and a half of Community unless you're one of us diehards. That's a lot to process at once for a show trying to pick up viewers.
they didn't have a choice, not many people even tune into just a half hour of the show, and it's better than the network not airing the episodes. the show has basically given up on trying to gain new viewers, you should know that by now. It's better off this way then airing them one a time for the last three weeks of May with zero new episodes from the other shows, no one would watch then other than die hards as well. sooooo
regardless it's not a risky move, it's just a necessary move.