From the distro's maintenance point of view, those two default applications did become a snap because that update model suits their purpose.
Welcome - for the latest documentation and translations.
Software Boutique - because it's a curated collection of software, and its database is stored within the application (it's not downloaded remotely).
Before, the user had to explicitly click a button to "subscribe for updates" (add the PPA) via either program, but new users might had missed it or misunderstood why. Updating the Boutique database is essential to stop software breaking because of external factors, such as Spotify changing their signing key. Some software isn't available from an Ubuntu repo.
Official flavour policies may had influenced this too:
- PPAs cannot be pre-installed (possibly for security reasons)
- Updates to the package had to go through a SRU process.
- That could take some time for someone at Ubuntu to review, but it could be rejected if it's not a bug or qualified for updates.
Upside of snap: delta updates - could be as small as a few 100 kBs rather then downloading a new 30 MB package.
Debatable: auto-updating
Downside: Welcome doesn't appear straight away in a live session or after a first boot! And that non-standard ~/snap directory in the home folder. 
Best wishes to you too @Algy, maybe we'll see you hop by again 
A few members stick around here even though they don't use an Ubuntu implementation of MATE.