Having lived through that era, Frames (and Framesets) were mostly killed by the proliferation of PHP on cheap hosts. A lot of Frames were used for building navigation elements without a lot of copy/pasting between HTML files (headers, footers, navigation bars, etc). The alternative to frames was either copy/pasting or ancient early static site generators. When PHP became common and cheap most people moved to server-side inclusions. These days we've come full circle back around to static sites but not returned to frames simply by how much static site generators have either become more common and/or easier/better to use.

> When PHP became common and cheap most people moved to server-side inclusions.

You don't (and didn't) need PHP for server-side includes. Apache has had them forever. A lot of cheap hosting is LAMP stack, so has PHP available, but you don't need to use it to get SSI.

https://httpd.apache.org/docs/current/howto/ssi.html

At least from my experience it was easier to get PHP than SSI - often if you were getting SSI, it was in conjunction with basic PHP support.

Mind you, I'm talking about cheapest hosting possible, where 20 MB was a "step up" option.

From my experience SSI was not enabled on many hosts. If it was you usually had CGI or PHP too. Or maybe even mod_perl.

SSI was commonly enabled on university web servers IIRC, but not commercial hosts.

I did use SSI some. My impression at the time was that cheap hosts that rolled out Apache services and had SSI enabled also generally had PHP enabled and PHP was the "better" choice at that point because it was both more powerful and more portable given Apache's SSI was unique to it and not supported by other hosts (then or now).

Frames made a side or top nav clearly separated from the content in the middle. Framesets were super useful, in that regard, in the era of dialup.

>but not returned to frames

also, CSS Flex