I think the big thing PHP offered was people could easily deploy their project just by copying files to a directory. A link directly to the file got dynamic content. Most shared hosting also put index.php in the default indexes to check for directories. So it was simple to have clean looking links without using a rewrite module.
Deploying to the cgi-bin directory was more convoluted and error prone for end users. Even when you did it right all of your URLs had an "ugly" cgi-bin component.
The easy deployment and cheap shared hosting with their managed MySQL instances really helped boost PHP's usage.