Why? Php wasn't some new, amazing idea. There was perl at the time as well. Php took off mostly because of mod_php, but there was already cgi. There was also asp from Microsoft that had almost all the same properties as php e.g. inline code and markup, upload and refresh), but Windows only. Php was just a little simpler to configure on a Linux webserver than cgi, but at the expenses of using php -- we've been paying for the decision for what, 20years now.

As someone who works in php daiy, I despise the language. It's getting much better than php4, but it still is lacking.