This is probably gonna get ignored as usual. But might as well voice my opinion.
Well lets face it guys. Unlimited Adventure came at a time when the anime boom had run its course and One Piece was coming off of the mis-management that was 4Kids. It had little fanfare (heck in fact I don't even remember any advertisements for it at all outside word of mouth) and people still had the 4Kids dub on their mind.
Now think back to when shows like Zatch Bell and Yu Yu Hakusho we're on TV during the height of Toonami's run. They got games imported or made due to the fact people we're familar with the series. Yes we got the Grand Battle and Pirate Carnival games. (Not the best games but hey better then nothing. We did get the awesome GBA game though) but that was 4Kids marketing at the time and, as much as I try not to bash them, their dub wasn't really gaining much of an audience. And still even then, OP was in a shift of networks and the audience that wasn't familiar with the show i.e: the majority, were either lost since the show was in the middle of arcs or didn't think much of the series.
Granted it was great to hear the new voice cast in UA and what (hopefully) will be the future performance of later characters. But these little tidbits barely helped anything. Only the fans of the series would get a kick of this and that was a very small group. Hardly enough to push the sales over the point where Bandai will consider bringing more OP games over.
It wasn't so much as bad sales but rather just bad timing. Had this came after the first few Funi DVDs, then yeah it probably would've had better reception. Still it was a miracle it came to the states at all.