Honestly, I think it's a bit of everything that hinders her as a jungler.
First off, her CC isn't exactly that great. A slow isn't bad, but a lot of the characters played at top level have gap openers or escape mechanisms. Tristana is going to rocket jump away, Caitlyn has 90 caliber net, Ezreal has arcane shift, Elise has rappel, and so on. Compare this to other junglers... Nautilus has his hook, Zac has a stun, Fiddlesticks has his fear, all of which are simply more reliable. This doesn't mean you can't get ganks, but it does make it more difficult.
Secondly, Kayle can fall behind irreparably. Her Q and W each cost a lot of mana, and if her blue buff gets stolen, she's in serious trouble. Sure, you still have some utility with your ultimate and heal, but Kayle's not a champion that has an easy time catching up when behind in levels/gold. This is particularly bad for a jungler, simply because they take more risks and get less farm than a champion in lane.
Third, her ultimate is only particularly good for one person tower diving. If they can outlast it, or simply run past their tower, it's not going to particularly help when ganking a lane.
In short, compared to the top level junglers, Kayle is a champion with sub-par gap closers, CC, and a non-damaging ultimate, and if she falls behind, will end up being utility-only. That's not to say she can't work, but there's simply too many risks and not enough benefit to bring her out in top level play.