Dunkey exists in his own space, many have tried to do what he did and failed (he even made a video about that).
There are few YouTube comedians that are legitimately funny like Dunkey is. There are others with their own shtick like Zero Punctuation but most get stale after a while.
Dunkey always manages to keep things fresh. Plus there's the part where Dunkey is not only funny, he's a very skilled gamer as well.
Usually video game comedians have to rely on three things:
The footage of the game needs to be broken or funny, players need to play along with their jokes and they need some kind of gimmick.
Dunkey rarely needs all 3, because's he has perfect comedic timing and can turn something that's not funny into something funny.
That makes him a real comedian who happens to play video games as opposed to someone who plays video games and jokes about them and that's why even his videos without any gaming content work in tandem with his absurdist humor.
It is also why Dunkey unlike other YouTubers like Jim Sterling doesn't have to rely on being overly negative or critical.
If Dunkey had a wider range of interests beyond video games and pursued a career in comedy he probably would've had his own Comedy Central show or tour by now.