Mainly because he hadn't died yet. DC;'s next animated movie is an adaptation of Grant Morrison's 2006 Batman & Son comics, which introduces Bruce's League of Assassins-trained kid to the DCU, and has Batman facing one of his most dangerous adversaries: parenting.
Huh. I have no idea why DC decided to follow up its New 52 universe-creating Justice League: War feature with this — other than a desire to release anything with Batman in the title, obviously — but it does look pretty good. Damian sounds less precociously badass than I imagined in my mind, but I can handle it. Especially if this is a set-up so DC can reveal movies based on Morrison's Batman and Robin run later this year.
Here's the voice cast, courtesy of ToyNewsI:
The voice cast for this one includes Jason O'Mara as Batman/Bruce Wayne who also did the voice for Batman in Justice League: War. Stuart Allan will voice Batman's son Damian. Morena Baccarin will voice Talia who is Damian's mother. Giancarlo Esposito will voice Ra's Al Ghul, leader of the League of Assassins and Talia's father. David McCallum will voice Alfred Pennyworth, Xander Berkeley will voice Kirk Langstrom Man-Bat) and Thomas Gibson will voice the character Deathstroke.
Son of Batman will come out later spring, apparently.