They're pretty far into the development. I wouldn't be surprised if they already had the engine locked down, and the only changes being made were those to correct bugs. I guess it all has to do with how much spare time they have for frivolous things like animated capes. That's not to say it isn't doable, because it certainly is, it's just that adding things later in the development of a game can cause other things that worked flawlessly before, to break. And speaking of things that worked flawlessly before, there is the issue of going back and changing pre-existing capes and blah blah blah. That being said, 8 months is, in fact, 8 months, and it all depends on the amount of time they have.
tl;dr: It's a small feature that is probably too trivial to be added in, because it's too late in the development phase to implement a potentially bug inducing feature that would require the modification of the current engine.
Oh, certainly, if it's not there now it's unlikely to be there ever - but it's not not there because the engine doesn't support it, the engine doesn't support it because it's not there.