Rather, it was popular software like bulletin boards and blogging platforms that built the demand. PHP used to have one of the lowest barriers to entry because you could get by with plain HTML and incrementally add business logic inline.
Not quite true. With shared hosting it was (is?) uncommon for a user directory to have ExecCGI enabled. If you wanted scripts to run they had to live in the cgi-bin directory. Additionally mod_rewrite could be expensive on low powered servers. This all meant doing anything dynamic meant "ugly" URLs and meta tag forwards if you were on such a shared host. It was also non-trivial amounts of effort to get some random CGI script working since you needed to know enough to get the shebang path correct for the server and set the right permissions.
Contrast this to PHP where you dropped a .php file into your user directory and you've got some dynamic content. Platforms built on PHP became popular because you could upload them to your user folder and they just sort of worked. There were no special executable paths, no shebangs, and no execute permissions to set.
Perl was huge in the CGI space for a long time but the (consumer) content platforms built on it weren't nearly as successful because of the difficulty of mere mortals getting them running on their shared hosting plans.
There was a reason that in ~2000-2005 you could find PHP shared hosts for $1/$2 month, and that Python/Perl/etc. shared hosts were much harder to find and more expensive. People started using PHP bulletin boards and blogging platforms because at the time it was easier and cheaper to run, but that's an effect and not a cause.
Chroot, containers/jails, SElinux, these are all OSwide and would protect PHP just as much a Python or nodejs. There’s a reason PHP is dying off.
PHP made "deployment" very simple. On the other hand, Python is complicated to deploy even today.
Not that I would ever recommend using as a templating language in 2021 :) but it’s cool that it can do that without any external library.
Python is great if you want to run your own server or VM per site. It's not so great if you want to run a shared server and it's shared servers (not VMs) that brought the price down and therefore opened the floodgates of allowing people to run their own sites cheaply and easily.
Is there anything like php or bml (bradfitz's equivalent for perl) for python, so that you can put code right in your html to be replaced at serving time with the code's output?