I think it was... SHTML? that allowed for server includes. My recollection from... 25ish years ago was that it was generally quite well supported and worked quite well (and was dead simple to implement). Not sure why, if that was the issue, the fix didn't quite catch on (but it's totally possible I'm mis-remembering the state of browser support).

Server-side includes worked fine but weren't enabled by default in any of the mainstream web servers. I think the lack of default-enabled status hampered their adoption. Joe User couldn't just FTP a bunch of ".shtml" files up to their shared web space and expect it to work right.

I certainly used the heck out of them in the late 90s, though.

It would have been very cool if HTML had been created with the ability to do client-side includes without having to resort to using a Turing-complete VM in the client to do it.