My assumption is it's for reducing the number of things they need to configure, and therefore troubleshoot.

It's easy to say "The newest Kali release is the distro the org will use" instead of "Use whatever Linux flavor you want and here's an install script that may or may not work or break depending on your distro and/or distro's version".

Them spending time troubleshooting a setup that's out-of-spec is still time billed, so it's better for their customers for everything to roll smoothly too. They also just want to execute their job well, not spend time debugging script / build issues.