Because they have to pay the themes they like all the time...........just imaged a new user go the WB gallery seeing 9 out of 8 are not free as of now,I do not think that encourage considering he/she is just start having the experience. Yes,there are tons of free skins over there,but I doubt as a new one he/she will has the patience to rummage the entire gallery looking for them at the first time.
Gaspershooters, being that you're not a Master skinner, you don't know what people are willing to pay, or not. We as Master skinners we have a complete run down of all our sales, and also we can figure out who is a non-member of Wincustomize, so when they purchase a Master skin, they don't get the member discount, so that that tells me that those are new people to Wincustomize, so, I can say about 80% of my sales are from non-members, and we as Master skinners know who's willing to pay for a skin, so I believe your premise is wrong. Granted, there are those that aren't willing/can't pay, that's fine, but there are more people buying skins than you think! Also, I try to upload a free skin, out of every three Master skins I make.