It's probably just that men aren't expected to like or want a lot of color or pattern, so most of the market just go to solids/blenders/plaids for men's quilts.
In my experience guys had all kinds of tastes in fabric, but a pretty consistent preference for bold quilt patterns, like the zig-zig Peckish posted. The big impediment was more often colorblindness than some bias towards certain motifs or colors. If half the shop looks like shades of grey, then no wonder you just want simple prints and solids.