Although a lot of people might tell you it's because of turtling, I don't think that "stalemates" or seeing people attack a stable hold are all that annoying to watch/play. Often times it's really fun to see players attack a hold and looking at the decisions each of them make in any given team fight.
More-so, I think it's the fact that if a push fails on these maps, and their respective gamemodes, there's not much pushback. On 5cp or koth, you might be able to have a complete turn around in any given moment where the defenders now get a chance to bring it all back. It's more dynamic, and is usually much nicer to watch and play.
That being said, I don't think a lot of current a/d maps are that well suited for competitive, to begin with. Pickup placement, defined routes, and general balance seem to be all over the place for most of them. It might help if they were more streamlined and if there was a better way to test out more maps (looking at mapchamp), but currently the pool we have for them is pretty small.
Edit: Even bigger than all of that is the timer system for deciding rounds is inherently flawed.