they themselves said why: Disney is responsible for cancelling, burying, and destroying the records of a lot of shows that fans seem to really love. TAG, the largest animation union, has been raising awareness of extremely unfair treatment of the creative side of Disney and other companies for a while now, how years of their hard creative work has been squatted on without ever seeing the light of day.

apparently, the hackers don't really seem to consult the artists they're purportedly trying to help by doing this, because none of them want to see their work leaked like this. I think these hackers just selfishly want to see the materials behind cancelled shows they were looking forward to.