Because you don't wanna be the guy who broke up with the girl who has cancer.
But the girl isn't real anyway, he could've never said she had leukemia, said she recovered or never went to the press about the whole deal to begin with. If he were trying to hide his sexuality then the whole cancer story could've been avoided and a healthy fictional online girlfriend presented instead.
There's something incredibly strange about this whole thing.
The thing is, if we are under the belief that he's in on the hoax, the narrative around him doesn't benefit from his girlfriend not having leukemia (or her recovering from it) and him breaking up with her.
And you're right; this whole entire story is bizarre to say the least. If what he's saying is true and he was the victim, then he's a pretty awful boyfriend for not at least trying to force the hand of his girlfriend by coming out to see her while she's in the hospital with leukemia.