Because there is no "booo" button. Every post in medium just have Likes or none, you can like or comment just that, making a critical comment is to much for most of the users that just want to say "I disagree"
If medium has some kind of down vote, it will regulate itself a lot more, and users that disagree will not have to go to make a comment and expose themselves being critical.
Right now is full of "Content Hackers" trying to make reputation.
So long as an article makes a reasonable attempt to present a viewpoint, downvoting does not add anything to the issue. If you disagree, upvote those replies that you agree with (unfortunately, Medium makes that more complicated than is should be.) If no-one can be bothered to say what's wrong with the article, maybe there isn't much wrong with it.
For example, xmlrpc has been recommended in comments as alternatives to consider. Maybe what's wrong with Medium is that the informative replies are on HN... (to be fair, grpc is mentioned on Medium.)
If medium has some kind of down vote, it will regulate itself a lot more, and users that disagree will not have to go to make a comment and expose themselves being critical.
Right now is full of "Content Hackers" trying to make reputation.