I'd say the number one reason a lot of open source projects use PHP (Wordpress, Magento, etc.) is because that's what you can run on dirt-cheap shared hosting.
I'm not sure why PHP got in that situation, but I expect that to change with VM and Docker becoming pretty cheap. Now you can have a low-cost hosting of any language.
The "several reason for choosing PHP" surely apply to Python even more than they do to PHP?