The only why I saw was this comment This is one of the reasons why building on top of the old API wasn’t an option. There was very little separation of concern

The only why I saw was this comment This is one of the reasons why building on top of the old API wasn’t an option. There was very little separation of concern in the original API and MongoDB specifics could be found even at the controller level. As a result the task of adding another database type in the existing API was too risky. This seems like the problem was more related to the API layer code quality rather than where the data was stored.