I've worked in consumer product development, and it's been obvious from the beginning that the reason for not implementing chat channels is avoiding the (substantial) support cost associated with enforcing guidelines on their use.
Yes, WC3 and SC1 had chat channels, and yes, both were very popular games. However, World of Warcraft has both grown Blizzard's audience substantially, and has also set a particular standard of support that's been based on having a continuous revenue stream from the game.
It's quite likely that the choice with respect to chat channels came out of a realization that providing the level of support people expect today to deal with rule-breakers would require Blizzard to charge a monthly fee, and that the way it fell out was that not having chat channels was seen as preferable to charging a monthly fee to play the game, particularly since Blizzard's competitors in the RTS market do not charge such fees. Also, it's quite possible that Blizzard's leadership are unhappy, in retrospect, with the level of support they were able to provide WC3 and earlier games, given their WoW experience.
Now, in the long run it may be that Blizzard encounters such pushback on the chat channel issue that they basically have to eat the cost of that support. If this happens, I'm not entirely sure that the long-term result is what the community would want. That long-term result might be that the NEXT game simply includes a fee to play to ensure that the game remains profitable to support.
Blizzard is a very successful company, but they're already plowing the bulk of their profits back into new development and rapid growth. Their resources are not unlimited, and saying "Try to tell us you can't afford this!" doesn't make them so.