The highest profile previous series for Guarascio and Port was running Just Shoot Me!
for a little while (seasons five and six, it looks like). It appears they've been with Sony Pictures Television for years, so Sony must have decided that it was better for the company to move those two with an existing contract to Community
rather than renew Harmon's contract. Vulture
says that Harmon and Sony have had difficulties since the beginning of the show due to Harmon's management style (with the most recent public example being, of course, the Chevy Chase situation).
I'm disappointed that Harmon is leaving (especially since that article says no one contacted him about a new contract), and the news that McKenna is also leaving adds to that disappointment.