Bro. Its a known fact that Diablo 3 was developed 3 years ago only. They had sketches and concept art etc before that but most of them got scrapped.
It not 12 years lah. 12 years is the time interval between the series.

During that 12 years time, all of them busy developing Warcraft series/WOW and Starcraft.

And Blizzard isn't the same company we used to trust long time ago.
Gone are the days when Blizzard comply to "We will release it when it is perfected" philosophy.
You can see how "half-ass" is WOW cataclysm expansion and Diablo 3 is.

This massive drop in quality can be seen after they introduced the Battlenet 2.0 system. (Post Activision merge era)