The reason for why the backend for the snap store hasn't been opensourced has been explained multiple times. Namely that it would be expensive to open source it with little benefit in return. Canonical already spent a large amount of investment opensourcing launchpad and nobody other than them operate it. Mainly because the majority of the costs are for operating an instance which most other distros aren't willing to…
The Snap server's closed-source nature is not a benefit to users, and there is no excuse that can justify this from the users' perspective.