Fads and scarcity! StatTrak versions of skins drop less than those without. Then patterns that people like. I haven't checked CSGO skin sites for awhile but normally multi-colored "fade" skins will all-blue patterns were very popular about a a year ago. How beat up the skin is determined by a number between 0.00 (best) and 1 (worst), so skins with a very low or very high "float" are apparently more desirable.