One reason could be: "Picnic's creator has better taste."

Sincerely, Picnic.css is selling itself better. Min's site doesn't look very good. People aren't even going to care if its semantic or whatever. This looks particularly bad: http://minfwk.com/download.html - a tiny textarea and a giant button piled up, nothing is aligned, etc. And you're trying to sell me grids.

(Sorry for being so hard - I'm trying (and failing) to find a better way to write.)