There's a lot of reasons why Diablo 3 is up there for most googled game and 5th most googled topic. Some of them are looking up builds, strategies, drop locations, and more relatively neutral or positive things. On the other hand, there's probably a lot of people googling Diablo 3 to try to fix technical issues (most of them have been fixed, but there are still some issues with some users).
Also, I remember reading about this on reddit (I think it was reddit) a few weeks ago. I wonder how dated this is?
I wonder if "Diablo 3" includes things like "d3" and other related searches ... if so, then many will have contributes just by doing things like "d3 ice climbers" or "d3 barb" to get to the relevant blizz pages quickly.