There was a problem with the Wonder Woman embargo. Donna could only appear in the Teen Titan Go! comic series (and they were able to sneak in select cameos of her in at least ONE episode). The embargo ended or was at least lessened around the time of season 2 of YJ (where they snuck Donna in as a member of the team during the 5 year time skip if you look closely enough) and it also allowed her to have a prominent role in that SBFF shorts.
At least thats what I can recall offhand.