because if its a theme people like they will buy it regardless of if its *that* great or not... if its some made up theme whos to say its going to be good or bad?
people won't pre-order, they will wait till other people review it, and inevitably since its not a bally/williams it will "suck" no matter what
and thats why I would be willing to bet we will never see a "unique" themed pin, ever again
anyway, a lot of the most popular pins are themed... MM is pretty much a monty python rip off, TZ, AF, TSPP, IJ, etc... AFM/TotAN are probably the only non theme pin I can think of that are really popular...