[minimaxir]

Diablo 3 was far from a failure as it broke a number of financial records; the main problem was the game-design pitfall of basing a loot-based game around the Real Money Auction House. (which they fixed in the expansion)

In a way the Diablo 3 RMAH helped validate the modern freemium/gacha business model for mobile platforms.

[Ataraxy]

Blizzard has a huge fanbase that will legitimately buy up anything they release however good or bad it is.

D3 was given away for free with a year subscription to WoW at the time which skewed the numbers a bit too.