I think it's a combination of factors and that music, or rather the success (or lack of success, depending how you look at it. Not going into that now) was a factor. Gem's injury too, but not because it stalled momentum, but maybe because he couldn't or didn't want to continue, like someone else said.
Very likely that Liam's private life was a factor as well.

Anyway, those aren't reasons to split up, right? Could have taken a hiatus. And if Liam's private life was the thing that made them split, then a solo career definitely won't help.

Anyway, one can only speculate and I don't know, obviously.
Has any of them ever explained why they split?