Working with large amounts of persistent data is hard. Limiting the architecture to a single database system (MySQL in this case) generally makes managing and s

Working with large amounts of persistent data is hard. Limiting the architecture to a single database system (MySQL in this case) generally makes managing and scaling much easier, versus having to know/learn how to scale multiple systems independently. Even if Redis was a better fit for some of their use cases, it just makes it much easier to not have the additional persistent database system to manage.