I don’t think that http/3 is easier to implement than http/1.1 especially since h3 is stateful where http/1.1 is not. Especially not when everything should be working correctly and securely because the spec does not always tell about these things. Oh and multiplexing is quite a hard thing to do especially when you are also dealing with a state machine and each of your clients can be malicious.

httpdito http://canonical.org/~kragen/sw/dev3/server.s implements HTTP/1.0 GET in about 320 machine instructions, and I haven't yet found a questionably compliant client that doesn't work with it. Writeup in http://canonical.org/~kragen/sw/dev3/httpdito-readme.

Because, you can iterate on the spec with all the stakeholders without ever writing a line of business logic. There are tools which can create a dummy web server from the specification and you can build clients without implementing business logic. I thought the advantages of spec-first development are obvious but I hope I helped.