Debian is a great OS and still runs on some of my servers! However, what about the release/EOL cycle, which might affect some people's reasoning? Along the lines of: "I'll set this up and I want to keep it running reasonably for X years with unattended-upgrades."
Debian releases are supported for just a few years: https://endoflife.software/operating-systems/linux/debian
In contrast, Ubuntu LTS releases are around for about 5 years or longer (HWE): https://endoflife.software/operating-systems/linux/ubuntu
Technically you do have Debian LTS, but it's never mentioned anywhere as prominently like Ubuntu LTS is, which feels a tad odd (because it works fine): https://www.debian.org/lts/
Of course, many looked in the direction of something like CentOS for this, or RHEL, or Rocky Linux or Alma Linux etc., which are supported for around 10 years usually. In my experience, DEB distros play more nicely with most of the software that I want to personally run, though.
Eventually I persuaded them to let me use CentOS 6. What they didn't know is they ended up running Debian stable for the next 7 years thanks to a pact with the other sysadmins. Eventually it all went to AWS and Amazon Linux which is basically a uniquely fucked up version of CentOS.
Really I'd rather run FreeBSD but no one wants to pay me to do that.