* Hard to keep users on your site - in an RSS reader you just open the next interesting article, most likely from another site. So it reduces page views.
* Harder to serve ads - RSS readers show the text of the articles, not all the other stuff on the page of the publisher
* Easier to steal content - other sites have an easy way to take your content and republish it on another website.
* Harder to track your users - most rss readers just show the content, not all the javascript nonsense required to track the user.
* Harder to monetize - for profit sites like to keep their readers behind a paywall, but how likely is it that you'll pay for a new site, if your RSS feed shows content from 20+ news sites? You can't pay for all of them, so you'll most likely pay for none.
If you want RSS to succeed, it needs to bring value to the publishers.