One of the main historical reasons was the Debian project’s puritan approach to open source, meaning the distro was very picky about what it could easily run on. As an example, most network drivers for Realtek nics weren’t included out of the box as they contained non-free code, there was no direct way to install Nvidia drivers instead of nouveau, a lot of the hardware didn’t work in the installer unless you sideloaded the drivers from a usb stick and so on.

There was a non-free ISO version to get around this, but you needed to know of it to use it, and it wasn’t provided anywhere by default. The download page for it was just a barebone directory listing within the mirror. No link or information was provided for it on the main project page.

Starting from version 12 or 13 (don’t remember exactly) proprietary drivers have been included in the installation images, which removed the biggest pain point (IMO) for novice users. Apart from that Debian has been one of the easier distros to install, and has things like a considerably better experience when updating to the next major release. It’s not really slower to update packages than Ubuntu, as I’d be wary of recommending the non-LTS versions to novice users. They tend to be quite unstable compared to LTS.

Personally I’ve daily driven Debian for close to five years, on all my devices except the work laptop. That one is running Ubuntu 24.04 as the employer requires either that or Fedora for Linux users.