That's misleading.
1) Diablo 3 has been released on just about every damn console there is, Diablo 2 was just on PC. Many fans ought multiple copies.
2) Reaper of Souls was a steep drop from Diablo 3. and that number includes probably includes Reaper of Souls expansion copies. Using that metric, Diablo II sold 34 million.
3) Diablo II sold something like 17.5 million copies accoding to VGSales wiki (http://vgsales.wikia.com/wiki/Best_s...Blizzard_games)
4) Diablo II's sales were box copy, Diablo III were digital which alters and inflates exactly what is being measured
5) 'Botting' was a huge thing in Diablo III and China fora period of time, you had to own a copy for each bot, unlike in Diablo II, that inflates further.
6) No one plays Diablo III anyore. The Necromancer pack has been the only thing buyable since Reaper of Souls. If there were people playing they'd be selling things to them. Even StarCarft II has all kinds of DLC, and it's not exactly booming right now.
Initial sales are a small fraction of profit these days. Diablo III did not reach its potential financial success either, no one is playing. No one is buying. So it's not even a case of being outnumbered in the market, it's just that a lot of people bought Diablo III because of Diablo II, that only works once. Otherwise, they'd be releasing more shit. Diablo III was a failure, even financially (in terms of possibilities) and Blizzard's actions prove this beyond a reasonable doubt.
Furthermore, Diablo sold something like 2.5 million.