It's also a bit more complicated than simply promoting.
Those who care about rewards may fear being blindly downvoted. They would be noticed, but not for the content.
Then, multiple tribes have their own token sink to promote content within their trending. So, boosting to the general Steem public feel unnecessary.
I think some of the blind downvoting has stopped, also in the latest @minnowbooster post we mention that we have introduced a burn aspect to our service so it is less likely stakeholders will downvote a post upvoted by MinnowBooster.