For me it just doesn't scale beyond a certain volume of posts. It is great for 10-20 sporadically updated blogs but as you start going beyond that, for example by adding a critical mass of frequent posting writers or professional news sites or aggregators, it becomes stressful to keep up and the signal to noise ratio drops. For all their faults, Twitter and Facebook both do a reasonable job of assisting with moving good stuff to the top of the "river" of news.
Personally I've started using tools that sit one level above Twitter and show me things a certain number of my "friends" liked or retweeted over the prior 24 hours or 8 hours or whatever. (Right now that's Nuzzle app, previously it was news.me email newsletter.) I follow nearly 1000 accounts on Twitter, and while this is excessive it is manageable and I see things I'm interested in without much stress.
RSS could hypothetically feed into an algorithmic sorting tool but it tends not to be used this way. My theory as to why this is is that, at least in the text news use case, it has a small and opinionated user base and only a subset of that base would want their news sorted for them -- and many RSS users vocally do not want that -- so it never gets built and RSS never gets the chance to grow to a broader audience.
Another issue is that there are slightly too many choices. It's not trivial to 1> pick a reader and 2> pick what feeds you want to follow 3> figure out how to follow them. It's not that it's so hard it's just not as easy as say Twitter.