Because such comments are at this point inane and uninteresting. As has been mentioned ad nauseum, Slack and friends are winning because IRC is a pain and lacks really basic creature comforts like offline messages and anything but text.

I'm downvoting them as I see them for this reason. Anyone saying "Why not IRC?" to the question of "Slack alternative?" is being willfully ignorant at this point.

Same goes to those who accuse people of astroturfing absent evidence other than "$ideaILike is being downvoted".