Chromium is maintained by only one person in Debian. This person is currently unavailable. Debian does not have good procedures to handle unavailable maintainers. There are also older reasons why there is only one person. In the past, it was a two-person effort.

Ubuntu moved to snaps, maybe just less manpower on building chromium dpkg's?

That's not the reason, Ubuntu still build deb chromium updates for 18.04, this is just a Debian policy or something else Debian specific