Yes you have to take times into account but DA was 2009, NWN2 was 2007. Also I'm trying to compare apples to apples, but lets look at something like Diablo 2 at 30m sold, this is more action cRPG. The point here is Oblivion was 1% of the population, if we add European Canadian, and US population, because they would be the primary group playing at the time. about 1B people. That is still obscure. You can find 1 in 100 people to talk to about it. With Diablo I could find 1 out of 33.
You also have to keep in mind, it is not JUST rulesets or how specific mechanics work that sell a game. If a game is bad, it will not sell well regardless of the ruleset. If a game is good, it will sell well. DA:O sold well (Asdie from brand recognition) because it was just a good game. It played well, was written well and it had good reviews. NWN2 was kind of.. uh.. bad. Not greatly written, it looked worse than nwn1 (and much worse than DA:O imo) and had poor controls. It was a huge dissapointment after nwn1.`Looking at Dragonage2, same idea. It was not received well and considered a bad game compared to DA:O.
Diablo2 was a very solid game, it was fun playing it and good reviews too. That all does help with sales, too. And Blizzard had the brand recognition to help too. This all goes beyond just rulesets.
