Whait a minute, there were a couple of Yakuza games on PSP, right? And the had people "walking around" in them didn't they?
WTH...?
You're saying that they did the series on PSP so why not 3DS? Well, the PSP game was static backgrounds with 3D characters (like the PS2 games, it was a Resident Evil 1-style engine.) So storage space could be an issue (except that 3DS games are supposedly 2GB standard and can max at 8GB, PSP was 1.8GB... this isn't one of those cases where Nintendo is using Giga
Bits to make things look bigger, right?) 3DS's graphics processor also supposedly puts its might behind efficient effects and layering rather than pure poly-pushing power (thus perhaps no Katamari on 3DS despite PSP and Vita versions?) and it might be possible that results would be underwhelming if they tried to do the kinds of city populations that Yakuza Black Panther had (even Lego City struggled to create open-world, tons-of-poly visuals on 3DS and that was a tentpole product.)
That being said, it seems plenty possible to do on 3DS. If it were important for the sake of Wii U entries in the franchise (or if it were the only way to get Black Panther 1+2 out in America,) I wouldn't mind sharing Yakuza with Nintendo's portables as long as Vita got its due.