I think it's because of who proposed it: the New Progressive Party of Puerto Rico, according to Wikipedia in the "51st State" article.

I like the arrangement, too, but I can see why its provenance might taint it in some eyes. That's unfortunate; it's a much more pleasant arrangement than the one we currently have.

I could also see people objecting to changing the US's hundred-odd-year tradition of having ugly and grating arrangements of stars. :) And the circular star arrangement _is_ something one tends to see flown by sinister US successor states like those in _Fallout_ and _Jericho_...