9000!!! In the PHP Dev environment, 9000 has been used for debugging for over a decade and the fpm-php project comes out and uses 9000 as their default. This is the definition of shooting yourself in the foot. Both solutions can easily switch ports but why choose 9000. It's not like xdebug was an obscure piece of software, it's the most widely used debugger on the platform.
It's just a pain in larger organizations that's all. I met developers in my life that have no idea how any of the dev env work, they just code, so if setting up docker dev instance fails because of their machine it require someone else involvement and modifications in docs for the future. It's not a big deal but it may be pain in the ass for some organizations.