Because debian and ubuntu are not trustworthy enough as server platform. They are not involved in the technical details of the basics, such as compilers, binutils, libc, kernel, systemd, gnome, wayland, servers, ... contrary to RedHat and Suse, who are maintaining these things. CentOS is the server standard. Which was the free RHEL variant, without support. 80% of the world runs this.

Debian and Ubuntu also have a horrible security track record. For devs behind a firewall it does not much harm, and they try to attract much more packagers with their focus on petty law and social issues. Which is fine. Most prefer the political issues, but some prefer the technical competence.