People in the comments grossly misunderstand what being "central" to a show means. They keep bringing up how he's popular with audiences and got a farewell episode. But you can be well liked and still not be "needed". He's basically saying he felt like a token gay :/
Bowen Yang opens up about his ‘SNL’ exit in Variety’s Actors on Actors, saying he never felt central to the show:
“I was like, ‘I think the show is in a great place without me.’ I never felt like I was that central to it, to be honest… I never played the dad or the straight-man teacher. I was always there as the seasoning.”