Why has Wit stopped animating for Attack on Titan and MAPPA has taken over for season 4?
Sooo here's some official statement, but it doesnt say much: "While Season 3 was in production, we consulted with WIT Studio, and it was decided that The Final Season would be created by a different production studio," Tateishi explains. "One of the reasons is because the end of Season 3 has the protagonist gazing at the ocean, marking a turning point for the story. Also, as creators, we wanted the anime to escalate even further. Through many talks with WIT Studio, we understood that the continuation would be difficult, so we looked for a new studio."
I was searching for more info, but it seems that for now we only have this short ambigious statement. We don't really know the reason other than WIT Studio probably dont having enough resources to meet producer's expectations.
Personally I think it's because the shift in after certain reveal is pretty big, also there's some massive scenes with titans in upcoming episodes. And WIT is great, but many times they had to use the CGI (like Rodd Titan, Colossal Titan). And this CGI looked pretty bad at times. MAPPA is great at implementing CGI. Its still noticeable at some scenes with titans, but its still probably better than WIT could afford. But keep in mind that's just my personal speculation.