Why Hasn't Universal Commenting/Discussion Caught On?

It really does seem like the next leap in social networking: Instead of (or in addition to) each site maintaining its own comment or forum setup, you could comment at or on any URL, and have your comments aggregated and threaded with everyone else's, all through the same service.

But no: We've got competing commenting systems (IntenseDebate, Disqus, et. al), in-house systems for WordPress and Blogger and so forth, all fighting for market share from the site operator side instead of the user side, and "universal commenting/discussion" doesn't seem to catch on.

A couple of years ago, I came across a site called Buzzbot. It appears to be defunct now -- not surprising since it was set up as a multi-level-marketing opportunity. It was one of those "operating system in your browser" projects, among other things.  The only thing special about it was its "browser within your browser." You could point that browser at any URL, and there would be a sidebar tab for discussion and commenting.

There was also a service call coComment -- also defunct, but from what I can tell it appeared to be a similar idea, implemented as a Firefox extension.

I think the idea is sound. In fact, I think it could be the next Twitter in terms of adoption.  If I were rolling it out (and no, I won't -- I've got neither the capital nor the technical ability), it would look something like this:

• A web site, a Firefox add-on, a Chrome extension, etc.
  
• Multiple account setup/login options ("connect with Facebook," "log in with Twitter," "use your Google account," "set up an account with us").
  
• When you go to a web page -- any web page -- with the add-on or extension running, or in an iframe or whatever from the service's site, you have a little tab on the side of your browser (for extra credit, let the user decide where the tab appears, maybe make it on of those that pops up when you move your mouse to the lower right of the screen, etc.).
  
• Click on the tab, a comment window opens. Bam, threaded discussion, and it's not dependent on, or controlled by, the site you're commenting on ... or is it?

That last question leaves open the possibility of a "premium" service that site operators can buy, which lets them exercise various sorts of moderation and control over the service as it's used vis a vis their sites. Or maybe not even premium. Maybe there's a default service, but any site creator can create a file along the lines of robots.txt that specifies certain parameters like excluded words, bans, moderation, etc.

Ad revenue seems like the obvious profit center.

What's the fly in the ointment? What am I missing? Too much overhead required to moderate from the commenting provider's side or something?

To me, this seems like exactly the direction I'd take Disqus if I ran that service. Why compete for site owners to implement your service when you could just offer it directly to users?