Giving a 25% portion of post rewards to comments as a handout was proposed for Steem v0.8.x but it was almost unanimously agreed by witnesses that it was a bad idea. It would have invited incredible amounts of no-value spam comments everywhere. Even without those incentives, a ton of bots showed up making generic comments in a ton of posts until people started countering them with downvotes.

I personally think if that change had happened it would have been a disaster for Steem(it). It would have allowed abusive/garbage bot accounts to scoop up a large amount of SP. Giving the bulk of rewards to comments is redundant and a handout. Comments can get upvotes already.