These things happen. Move on, nothing to mourn. Racing is marketing money made to look exciting.
In the US, BMW had an incredibly fruitful partnership with PTG - they won everything under the sun together. And just as quickly it ended, with BMW switching to Team Rahal Racing as a partner, now known as BMW Team RLL. I never read a reasonable explanation for this change.
In the case of Schnitzer, it seems to follow changing regulations in DTM. Can’t fight these. I personally thought that especially in later years the DTM was a dinosaur - silhouette racers with NA V8 engines that none of the manufacturers offer anymore to the public, and limited audience to the home country - not unlike a German NASCAR. IMO it’s time for that series to go, and for BMW to refocus that spending in other race series.
Just my opinion of course.