One size, so there is nothing to collect and no mix to get wrong. A pair sits at the cheap end of the stash range, so a parent or a visitor will pick one up. And nobody has enough socks.
A hoodie has the opposite problem on all three counts. Most students already own one by second year, you have to guess how many of each size to order, and it is the most expensive thing on the list. The full argument is in why to add socks to your university stash.