software-boutique is still part of ubuntu-mate-welcome - and its curated selection is contained within the application, rather then externally downloaded. Reasons that led to this becoming a snap include:
- New users often missed or misunderstood the "subscribe for updates" button.
- Updating
Welcome provides the latest documentation and translations. - Updating
Software Boutique provides the latest curated apps.
- Updating
- Updates were slow because they were subject to SRU restrictions.
- There's more involvement and review required to push to the official archives after release - it could be stuck in a queue or didn't meet criteria for qualification.
- Slower updates resulted in a less polished curation of apps - e.g. Oracle VirtualBox didn't provide builds for 16.04 till a week or two later (external source) or the Brave browser pointing to an old repository and expired signing keys.
- Official flavours cannot have PPAs enabled by default, and generally discouraged.
I believe Ubuntu MATE was the first flavour to provide a snap pre-installed, so I guess that was good feedback for the snap team.
Snaps are a mixed bag. 
As mentioned earlier I'm no fan either but I get why Canonical are invested in this technology. See this other topic for others thoughts:
(No, Ubuntu are not moving to snaps only - but chromium-browser will transform from deb to snap)