There's a lot of reasons people leave shows, so I could look it up or I could speculate, but I'm okay doing neither. He was cool though, I miss him. I hope they didn't kill him off just for the sake of manufacturing stakes for the Gorn conflict but that's because I hate that plot thread and wish it wasn't in a fight with Kirk over who's going to take over and ruin this show.
So, basically, they made his character to be amazing and lovable so that they could intentionally kill him off. They don't want people thinking that the crew is safe because it's filled with people from TOS. Also, they wanted it to affect Uhura.