Why does not green cotton hold up better?

So I was in the thrift store today looking for sweaters to wear, and sweaters to frog, not necessarily in that order, and I had to ask myself why green cotton sweaters do not hold up as well as green wool sweaters. There was something I particularly needed a Slytherin Green Cotton Sweater for, and no luck at all. They were all shabby.

But the people in Hogwarts do donate some nice things to the Red Crossed Wands. I got a man's extra large Shetland sweater with many colors - it will be a pain to frog, but I will be able to make a coordinated something with the yarn, and I may felt some of it... and there was a lovely green angora sweater I had to have for, you know, fuzzy snake purposes, and a sweater I am pretty sure is Cashmere that I intend to wear - well, at least till I get tired of it - HUGE green Merino laceweight that will yeild enough for who knows how many scarves and shawls, a little red sweater that looks cute on, a black sweater that looks soignee on - well, as soignee as a short dumpy body gets - and a peasant blouse that looks sort of like it came off a real peasant. Never know when you have to go out dressed as a Muggle, so those are really useful. I passed up a bunch of stuff too.