Because search engines (Google) and centralised blogging services (WordPress, Tumblr, Flickr, later Twitter, Facebook, etc.) offer better user experience and discoverability than webrings.
I dunno. Google, Twitter, Facebook, etc. don't actually provide what the web used to provide, so they certainly didn't improve anything from the early web. It's apples-and-oranges, but the apples mostly don't exist anymore. It happened as part of the shift the web made from being people-driven to being commerce-driven.
