Because it doesn't solve any problems except page loading speed. There are other things that people care about and the added complexity of implementing Layer 4 in Layer 7 make it even more of a monstrosity.

I put this in another comment in the above:

* Better authentication

* More secure caching

* Improved ability to download large files

* Better methods to find alternate downloads locations

* Making each request contain less information about the sender

* Improved Metadata

I brain-dump a bit here: https://github.com/jimktrains/http_ng