They don’t have to wait that long, it just worked out that way.
It doesn’t matter if they wait 6 months or two weeks, people will claim the character is too new to get into Smash either way. The only thing that matters is the character and other content it comes with avoiding spoilers(and it is very hard to spoil a SMT game after the trailers already show all of the surprises).
They don't have to, but the point is that
they do it anyway.
Sakurai just started being a lot more careful after Roy's game got delayed and rewritten to the point where Smash Roy and canon Roy are polar opposites as well as Mother 3 being Japan-exclusive, making Lucas an oddity (well, so was Fire Emblem in Melee, but that was an intentional move from Western divisions since Sakurai himself had no issue making them Japan-only if needed while Lucas in Brawl wasn't, iirc).
I'm not saying either of those things can happen to SMTV, certainly not. But I
am saying Sakurai has been playing it safe ever since, which is why new characters tend to not be less than 6 months older than their gaming debut. It's for the sake of accuracy because last-minute changes and delays can happen at any point, which can range from insignificant to completely rewriting certain aspects which can make Sakurai's portrayal of the character in Smash oudated.
Besides, it's not like Nahobino is the
only SMT option. There's Demi-Fiend.