Let's see.
"MORE MONEY" from some players in exchange of piss off veterans who support the game (and still are supporting) even when there were so tiny playerbase that the game has to go free to play.
Those skins are a token of gratitude that MSE give to the little amount of people that believe in the studio and support their project on hard times.
Put those skins aviable for everyone would place MSE on a very bad position, betraying what they promise a while ago.