I was there during the transition. Even rewrote a site from perl to PHP.

For newbies, not having have to write the "magic" string,

  print "Content-Type: text/html\n\n";

before being able to output anything probably felt quite friendlier and being able to mix HTML into the code, though it's not a good practice.

And the PHP standard library has quite a few things built in and combination of MySQL as LAMP was the hot word then, so I guess that's how it got traction and perl was gone from the web development.

I do like the Unix heritage of perl though. It does make you feel like a hacker when it feels the language has the shell syntax half baked in.