combination of reaching the culmination of this huge thing they'd been building up to for so many years, the loss of RDJ and Chris Evans (and perhaps more importantly, their characters) who really carried a lot of the pre-Endgame stuff on their shoulders and now having to rely on characters that no one really cares about to try and carry things, the Jonathan Majors stuff and having to totally shift gears as they were trying to ramp up the multiverse storylines, and fatigue of MCU/superhero movies due to so many coming out each year (which also started negatively affecting production quality, storytelling, and effects)