Nothing. Everything we create here at System76 is open source, either GPL or some form of permissive license.
(Edit: To be clear, I'm just answering your rhetorical question, not insinuating anything about the tone of your post for sure.)
I just figure Canonical probably has a specific vision for Ubuntu (just like System76 surely has for Pop!_OS), so they don't necessarily want to pull in every feature from every downstream distro. The prevalence of purposed-focused distros is one of my favorite things about the Linux ecosystem.
I actually haven't used Pop!_OS yet, but it looks good and it's on my short list of distros to try. I didn't mean to imply anything negative about it.