Ubuntu / Canonical were working on Snap for some years when Flatpak came on the scene. They’ve been shipping Ubuntu bits using it since 2016. In addition to the legacy, Snap is more versatile than Flatpak in that it can be used to package pretty much anything, including system bits. It’s also had a secure sandbox from the start. Changing to Flatpak would be a functionality downgrade for Canonical and Ununtu maintainers using Snap. In addition Flatpak can be used along with Snap on Ubuntu so there’s no need to not use both for whoever finds that useful. Snap lets Ubuntu ship software using less work, which means more up-to-date bits in Ubuntu. Users can install other software via Snap or Flatpak, whichever they find more useful.