As someone said above, it probably caused issues (freezes, slowdowns, crashes) and Ubi simply doesn't want to fund patching that. And why would they? Ppl buy new consoles, but when it comes to PC, they use machines older than first Playstation and expect miracles. Thus to avoid problem reports completely, devs simply introduced a limit in gfx game options depending on OS version.
Now I'm not judging anyone, their machines and OS they use. The best solution was to patch the game and make everyone happy. Square Enix would do that for example. I bet no matter how evil they are, EA would fund the patch. But in this case, it's Ubi we're talking about. The same company who's attitude is no matter singleplayer or multiplayer, you must run the malware during the gameplay or go away. Don't expect any kind of "friendship" towards players from them.
But not everything is lost. Modders are out there, sooner or later someone will find a way to trick the game into believing it's running on x64 OS. Or to allow higher gfx options.