[dontchooseanick]

Because you're supposed to use a single connection with HTTP Pipelining for all your ressources [1]

When index.html loads 4 CSS and 5 JS : 10 ressources in HTTP 1.0 needed 10 connections, with 10 TLS negociations (unless one ressource loaded fast and you could reuse it's released connection)

With HTTP1.1 Pipelining you open only one connection, including a single TLS nego, and ask 10 ressources.

Why not only 1 per domain so ? IIRC it's because the 1st ressource index.html may take a lot of Time to complete and well race conditions suggest you use another one that the 'main thread' more or less. So basically 2 are sufficient.

[1] https://en.m.wikipedia.org/wiki/HTTP_pipelining

[immibis]

HTTP pipelining isn't used by clients.