It appears that (at least according to a S15 DVD special feature) the commute between New York & where they shoot the show in California was getting too much for David so they made a deal/"came up with a formula" so he'd only appear in 13 episodes. I don't know if it'll be the same for the new season but I'd presume so

Mark says some nice things about him, including that he doesn't expect him to be gone gone & hopes that he'll show up occasionally. But also that he hopes that it gives others the chance to grow & have opportunities to do things they haven't before. Then also how David has been a mentor to a lot of the cast