Really, I don't think the team has a lot of say in the merchandise choices. The manufacturers (Reebok, Adidas, Nike, Zubaz, the Chinese kids sewing for WAL*MART, whatever...) choose the designs and which teams to sell and at what volume. There are also league licensing issues and I'm not sure if the team can just make whatever they want. For instance, I'm sure a pink Yankees hat wasn't Steinbrenner's idea.
eBay - redskins items in Caps, Hats Fan Apparel Souvenirs
There's plenty of nice designs of hats made for us, they're just not sold in the team's store.
Contract reasons with the uniform provider maybe?
edit: I just looked in the team store for myself and there are 31 different hats for sale...what do you want, caps with lights and helicoptor propellers on the top?