Surely the people who had purely performance problems with Perl CGI scripts moved to mod_perl? I didn't figure out when mod_php was introduced from casual searching, but given that mod_perl is only a year younger than PHP it must've been available to almost anyone who was considering rewriting their app in PHP. So I have to imagine there were additional reasons.
Wikipedia says that this source [1] claims early versions of PHP were built on top of mod_perl, but I can't access the archive right now for some reason so I can't confirm.
[1] https://web.archive.org/web/20130625070202/http://www.theper...