Nothing really. Distros have different goals. Some distros have more access to bleeding edge updates than ubuntu.
Some don't like that ubuntu forces you to use the snap store, which is proprietary, with packages that are larger and run slow and hog a lot of memory compared to other package managers (such as apt)
yeah, many packages now defaults as snaps in ubuntu.
firefox being a single but prominent example, the package on apt simply installs the snap now. You can get around that but you'll have to add mozillas repository. It's Canonical's proprietary thing so I guess it makes sense.