There are probably several factors at play, but the two biggest in my view are: 1) increased competition and 2) diminishing quality (technical and qualitative,

There are probably several factors at play, but the two biggest in my view are: 1) increased competition and 2) diminishing quality (technical and qualitative, e.g. in terms of release management, story telling, etc.). Examples in my opinion: Diablo has always had repetitive gameplay, but the stories/lore seems to suffer with each release. They are either working on SC3 in secret or have given up on the franchise. Their management of WoW Classic and rollout of releases has been awful. Without fresh progression servers, Classic is going to truly die in a couple of weeks with the release of TBC.