My assumption is that they make an educated guess on how it will sell and determine if it's worth it for them. Unfortunately, Universal doesn't seem to think much of anything is worth their time because they have released very few TV shows on Blu-ray. The only shows of theirs that I can think of on Blu-ray are Battlestar Galactica, Heroes and the last few seasons of The Office (there certainly could be more). Two of those shows are sci-fi shows and that's a genre where the fans can be counted on to spend their money. The other is their most popular comedy in a decade and they haven't even worked their way back to the earlier season of the show.
I think by now that all current tv shows should be offered in blu, if they're offering them at all. Some other hold-outs: Revenge Gossip Girl Private Practice Pretty Little Liars Nashville Dallas All the CSIs Warehouse 13, Alphas (and other SyFy shows, the absence of such genre shows on blu baffles me. Most of the shows, revealed here, seem to be of the female-demo drama genre, and perhaps they think women aren't interested in blu ray?? That still doesn't explain why the SyFy shows aren't on blu, but then--those are from Universal).
I think the only shows that Sony has released on Blu-ray are Breaking Bad and Justified so if they haven't even released Seinfeld on Blu-ray yet, I'd be shocked if they release Community on Blu-ray. That being said, I love the show so I'd happily buy it over.