The simplest answer I'd give you is that Rockstar do a better job of it. Story telling, character building, and the tasks that they set you out to do. They've been doing it longer, and are more comfortable in trying new things, so they are less carbon copy sellouts of the previous iteration that Ubisoft games tend to be. I too, got bored of AC after 3. My fav was AC2, but the current games are nothing like it.
Red Dead and GTA feel more fresh, and not just because they're spaced out wider. AC and Far Cry feel stale pretty quick. Primal was probably the only departure from the formula, but either you're into it or you're not. In fact, I'm more inclined to play the next iteration set in Montana because it looks more like what I enjoy about Far Cry games than Primal is.
Max Payne 3 was actually pretty well received. I loved it personally, apart from the Social Club which caused more problems than the game was worth in the end (I barely managed to finish it the first time round, but crippling bugs regarding sign-in ruined my second playthrough).
I didn't follow the hype of Watchdogs 1 so much, which is probably why I had an okay time with it. A bit cringy, but still rather playable and fun at times. Not fun enough that I needed to get Watchdogs 2 though, seeing Ubisoft's track record with dull-ish repetition.
The other reason is also because Ubisoft are some of the biggest douches in the gaming industry, that should need no introduction by now.
