Nearly all free webhosts run PHP too.
The barrier to entry is very very low
This, absolutely this. There's literally nothing to configure or setup beyond PHP itself. You open a text editor and write your script, save the file, drop it on the server and it's immediately ready to run. No messing around with convoluted toolchains, massive frameworks, or some elaborate "build process" just to get a "Hello world" to work.
I'm not sure if it's even a majority of users, but PHP certainly appeals to those of us who like writing plain HTML in a text editor and being able to immediately refresh the browser that has it open to see the effects of changes.
The language itself has its warts, but they're easy to avoid and IMHO the disadvantages pale in comparison to the extreme simplicity of getting started and continuing to work.