That's because Chromium is hell to keep updated in a stable distribution release apt archive. Upstream adds new dependencies that aren't packaged in the distribution's stable releases, for example. It's not practical to do anything but a whole load of bundling (in debs) to solve the problem. This then isn't much different to a snap any more anyway, and packaging a snap takes a fraction of the effort.
Anyone is welcome to maintain chromium as an apt package for your distribution release of choice, and you're welcome to point apt to that repository instead. The reason it's not happening is that it's increasingly impractical to do while also maintaining Ubuntu's quality standard for debs.