In the "transition" from FTP to HTTP, the level of abstraction in popular use has shifted out of the protocol and into resources (mime-types) [1], rel types [2], server logic [3][4], and client logic [5].

In the past, I've said that this extensible nature of HTTP+HTML is what made them so successful [6], but once specialized protocols began to falter, tunneling other semantics over HTTP became not just a niceity, but also a necessity (for a diverse set of reasons, like being blocked at a middlebox, being accessible from an the browser where most people spend their time, etc).

[1] http://www.iana.org/assignments/media-types/ [2] https://www.iana.org/assignments/link-relations/ [3] https://wiki.apache.org/httpd/DirectoryListings [4] http://nginx.org/en/docs/http/ngx_http_autoindex_module.html [5] http://stackoverflow.com/a/28380690 [6] https://news.ycombinator.com/item?id=12440783