Weirdly, the decision to use PHP was justified back then by the easy deployment of the tarball on some shared webhoster. Open yourwebsite.com/owncloud, enter your database credentials, and you already have your instance running.

Now that use case is gone, everyone is expected to use their big Docker image, but the server is still written in PHP. I can't shake the feeling that some people just don't want to give up the tools they are used to and have stuck with them for a bit too long.