It's odd because the costume discs are very popular. Releasing a 4-pack of costume discs with Marvel Battlegrounds would go over well and MCU Captain would have fit in with that.
But I understand that Disney Infinity is first and foremost a way to get sales synergy with Disney's new releases, so I suppose this would have been hard to avoid with Marvel. Netflix's Daredevil wasn't exactly for children.
Now that you mention it, a costume disc pack seems like an awesome idea. I suspect many people will pass on new-Cap (at least for a while) since they have the original Cap but I'd buy an MCU costume pack.