Many skins are limited edition. A few were offered as part of a subscription campaign, event skins may only be offered during the event - after that you need to get them from the market. Skins are periodically retired from the store. Scarcity tends to add value to skins on the market and better tools allow CCP to produce far more sophisticated skins than the basic 2 color ones we were getting a couple of years ago.
I expect this pattern will continue - increasingly sophisticated limited edition skins that are only offered for a short period of time - after that you need to pay market price - if they are available at all.