Because the number of copies sold clearly reveal the quality of a game... riiight. If you judge a game's value only under that perspective, I feel sorry for you.
It's undeniable that Diablo III rided on the coattail's of its predecessors, Diablo I and Diablo II. Let's us see how many copies a Diablo IV game will sell, in the future, while people still have D3 on their minds...