Thanks for bringing that up. Im wondering why they went through all that trouble though. Are there no alternatives to GA?
I'm not sure that would still be the case if the decision were being made today, and would quietly hope not, but I guess we can charitably say that the reason now is "inertia".
Personally, I think they may have underestimated (or failed to fully predict) the anti-google, pro-privacy sentiment in the wings, and it's clear even from this thread and the issues on bugzilla that it's probably cost them enough privacy-capital at this stage to have justified the extra work required to self-host.
But hindsight is 20-20. There are sunk costs now which also must play into the decisions.
The same argument applies to the whole of Firefox. It's more work and it's a lesser product. If Firefox can be a better product, than Mozilla Analytics could be too.
At this point it's clear that Mozilla is a business (with well paid management and staff) like Google that is using Privacy as a promo like Google used Don't Be Evil. Mozilla might be better in practice today, but it's not on a principled foundation. It looks like a Google Lite - Firefox vs Chrome, Rust vs Go, etc.
Or maybe the "anti-google, pro-privacy sentiment" isn't really all that big. Could be a relatively small but vocal set of people.
There is Matomo (formerly Piwik): https://matomo.org/