Because the same snap can run on releases of Ubuntu all the way back to 14.04. 14.04 has VLC 2.1 in the repo. 16.04 LTS has VLC 2.2. The snap store has 3.0.1 in the stable channel, with 4.0 (dev release) in the edge channel. Versions that will never be in the archive of those older releases.

As LTS releases age, the contents of the repo age with them. PPAs are one solution, but they're undiscoverable and not straightforward for new users to setup. Ubuntu has a ton of users who are 'sticky' on old LTS releases.

This enables the VLC developer to have one package that targets millions of users across lots of releases of Ubuntu - and other distros too.