[defanor]

I blame the bloat.

In some high-level languages even BSD sockets (and many other POSIX functions) aren't in the standard library, and there are various wrappers to provide "ease of use" and integration with a language's runtime system; plenty of complexity (and alternatives) even at that point.

RFC 2616 (HTTP/1.1, 1999) may seem manageable, but it's much more than just posting data and getting a response, and IME many programmers working with HTTP aren't familiar with all of its functionality. Then add TLS with SNI, cookies, CORS, WebSocket protocol, various authentication methods, try to wrap it into a nice API in a given language and not introduce too many bugs, and it's rather far from trivial. But that's just HTTP/1.1 with common[ly expected] extensions.

Edit: Though I think it'd also be controversial to add support for particular higher-level protocols into standard libraries of general-purpose languages, even if it was easy to implement and to come up with a nice API.

[dragonwriter]

> RFC 2616 (HTTP/1.1, 1999) may seem manageable

But it's probably not, as it's underspecified and ambiguous, which is part of why its been replaced as the HTTP/1.1 spec by RFCs 7230-7237 (2014).