I think it all started with WoW, when they realized how much more money they could make by going for things with the most mainstream appeal (which is why they now have Overwatch and Heroes of the Storm and such). It's way more bang for their buck compared to making a true improvement on a classic like Warcraft. Many other entertainment companies have realized the same, once they've gotten big enough.

Starcraft 2 is still quite good, though, IMO, and they still pay a lot of attention to keeping it as balanced as possible. I still find the high-level tournaments very entertaining to watch.

Things did change during wow. Three of the top guys left to do Guildwars. Blizzard North slowly collapsed after D2 and wow. Now blizzard is more the Anaheim guys than anything else.