Well, still (or perhaps even more so) - if they considered something like chat rooms to be as essential as many users feel, it would have been easier to implement earlier rather than later, when there are more products to support.

Then again, something like chat rooms could be implemented per game, minimizing impact on the entire battle.net system.

It just wasn't seen as a priority, or there wasn't the political will to push it through, or there's something else we don't know.