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.