What decides which skin gets made?

Serious question. Why does riot make the skins they make? I'm thinking most of the money they make is made off skin sales. The boards have long suggested skins that would have huge popularity. Popular skin = more sales = more money.

I don't understand how things like cat lady yorick, deathblossom zyra, lifeguard/pizza sivir, or dragon swain which have big followings get passed over for skin ideas like death blossom kha'zix, definately not velkoz, infernal diana. Not that these are bad skin ideas but it seems like something that might sell was chosen over something that would definately sell. What is the metric getting these skins out the door first?