I see that https://github.com/javascriptdb/jsdb-server is licensed SSPL instead of something more permissive like MIT. Could you explain the reasoning behind this? How does this affect my ability of self host?
Sure. You can host it yourself for commercial purposes and you don't have to open source anything in your product, just like you can with MIT.
The only difference is that if you want to create a service that offers JSDB as a service (compete with javascriptdb.com) you have to open source it. The idea is to make big cloud companies contribute back.