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 spend. Not even Mint operate run or operate their own launchpad infrastructure. They experienced large backlash back then, open sourced it and nobody contributed.
The same problem applies here. Snap store specifically from what I gather is a bunch of operational machinery that doesn't make sense without also operating launchpad. Such a cost operating would benefit nobody but Canonical because nobody else is bothering footing the bill to run an instance.
Second aspect that they wanted to avoid was the same pitfalls that they experienced previously with ppas and now being experienced with flatpak. Namely they want one location to find software, and one location to serve software. If users have to use the command line to add a external repo that has unfetted access, then that defeats any usability gains. That and the whole aspects of malware/trust goes out the window.
I will remind people that the most popular PPA to this day is a Java PPA being run by some 3rd party that doesn't offer Java. That PPA has root access to thousands of machines.