Because:How do you figure it's obvious? Given the sequel to The Dark Knight Returns features this character? Not to mention given the connections rumors circulating around Harley and The Joker being estranged relating to another character.
It's shocking to me that they'd go this route especially considering that there was already an established character to serve this purpose.
1) casuals/produsers wouldn't want Robin in their JLA stuff;
2) Robin's death made Batman "retire" and become brutal in TDKR;
3) casuals/produsers don't care if it's Grayson or Todd.
Or something like that...