At this point I have to assume the underestimation is a generational thing. Just going to Barnes and Nobles and malls in LA last week, and it shouldn't take an MBA to realize how popular and pervasive Asian culture is in the area.
The fact that Universal's big expensive mazes for this event were Star Trek, Back to The Future, and Dungeons and Dragons, but the only thing they could do for One Piece was a few meet-n-greets in a corner, suggests to me that the creators weren't really acquainted with what modern anime fandom is, likely because they have not interacted with the equivalent spaces that existed in their youth for things like Ghostbusters or D&D or (insert 80s youth touchstone here).