Give us a link to per-month selling statistics or your assumption is invalid. I don't believe that it "sells well" after 12 month.
Also What makes you think that bad things can't sell well?
It's wow and d2 that are selling this game, not d3 itself.
If this game was released with different name from different developer, it would not sell even 1/10 of what it was able to sell.
You never know what you are buying before your bought it. It's not a car, there is no test drive.
And most people don't even try to do a research before buying games. "Hey, I heard about this game somewhere" or "I saw an advertisement on tv". And then "well... it was not what I expected to see ;(".
The number of sales is not important. What is important is customer satisfaction. And the number of disappointed customers was extraordinary high.
No blizzard game caused so much hatred before.
Obviously the game is still better than most of the games on the market. It's just failed expectations.