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They did capitalize on the modder community in World of Warcraft (almost by accident, thank you Sam Lantinga! and what a success it turned out to be).

But what you need to understand about Blizzard is they are extremely conservative about the game experience. They're proud of their work and while they never consider it perfect, they want the one experience for all their players to be consistent and to have the consistent feel of Blizzard polish (which lacks with mods). They're the Apple of game design.

It's also a bit of a different question when you're talking about pre-wow and post-wow Blizzard. Pre-wow it's also a matter of "it's hard". Nowadays it's easier, and their philosophy is I believe slowly changing.