Paleo diet and its restrictions.

I have been reading up on the Paleo diet, and I am curious as to why corn falls under the no, no category. I understand that the diet wants to follow the “caveman” eating style, but I mean, what is so bad about corn? More specifically, what are the scientific notions behind this claim?
-air popped popcorn
-corn on the cob
-cornmeal
-CORN
what makes corn or any other grain under the paleo diet a restriction