Hmm....very hard to imagine why. It's been announced for several months, and it's not like it was announced before the sales numbers for ARMS came in (for the good and bad that might imply, 1.85 Mil). If it was transitioned to a smaller scale project with fewer issues to publish then a delay hardly makes sense, but also, expanding the project doesn't strike me as a profitable idea.

I know some people will hope that it's being timed around new content for ARMS, I just think it's too late for DLC, paid or otherwise, yet a little too early to see a Deluxe/Super type of edition with new content. We do have a new Nintendo on our hands, of course.

I could even imagine they just wanted to avoid the Smash Bros hype train, but since I think that game is going to have weekly DLC (items, stages, events, trophies, fighters) for many months to come, there's really no avoiding it.

EDIT: And I think the director of ARMS basically revealed that he has yet to be directly consulted about ARMS having a character in Smash, so I don't think it's an attempt to ride the hype train either.