To be honest I think it can just be simply put as they were no longer a boyband while still trying to exist as one. Marvin's becoming a dad and they all just looked older than any of the other boyband membersout there. New boyband fans are either going to go for how exciting being a One Direction fan must be with how big they are and all their media coverage etc, the 'edginess' of The Wanted (whether or not you think they are actually are is another matter but at least to me that seems to be their intention) or the niceness of Union J with George especially having the most 'cuteness' and sweetness of any boyband member out there at the moment imo. It also helps he physically looks by a good couple of years the youngest of any of the members even if he's older than the majority of One Direction. Meanwhile the original JLS fans will have probably moved on from the whole boyband phase for the most part. You could tell by tumblr, especially, that there was a great shortage of fans with the general characteristic of what the standard of boyband lover currently does. Very few gifs, no bromance shippings etc. It was quite stale from what I observed, much less activity than those three.
So it was the case I think that they just couldn't compete with the younger boybands but I do agree that it was no reason to split. They could have adapted themselves (or at least made the attempt) into a 'manband', producing maturer songs and distancing themselves from the typical boyband fare (why were there still dolls of JLS members in shops on the same shelf as dolls of One Direction, boys in their late teens/early twenties?). It's sad that they didn't try it really, because there was opportunity for it in my opinion.