The sales are probably decided on what champions and skins are in demand, if some one wants a champion or skin, they wont put it on sale, as the want is high so they get more sales, while if the champion/ skin is unloved they put it on sale to encourage the consumers to buy it. Makes sence if you think about it economically.

And with 34 millions player i dont think they give a **** about trying to please everybody, but that is aside the point in my opinion