Because they're going to a larger lineup with him on the floor (because of injuries) and the Raptors have small guards and Siakam also struggles against bigs in the paint. It's a catch 22 for the Warriors... they prefer the small ball lineup but that's also the lineup the Raptors prefer
Raptors lack the ability to put Boogie in the blender on the pick and roll.
I couldn’t believe Lowry couldn’t get by him. Boogies as zero lateral quickness, but Raptors don’t have any guards that can abuse him.