The really really short version is Elon wanted FSD, totally done, really fast... and Mobileye said "Yeah, there's no remotely safe or reasonable way to do that anytime soon" and it kinda escalated from there into a messy breakup that left AP1 cars largely abandoned for significant further updating (in part because Tesla never had access to substantial parts of the system only mobileye did), and meant it'd be a couple years before Tesla internally was able to get AP2 caught back up to where AP1 already was...and still years behind Elons intended target dates from back when the breakup happened (though it's reasonably far ahead of AP1 nowadays in most circumstances at least)
