I believe the R2 was a pretty primo model of astromech droid.
Think of it like a flagship SLR camera body. It may not be the newest in the lineup, but it is still the full featured beast used by the pros. The R4 or R5 units are the consumer grade models that get updated every year and may actually be more recent and contain some new functions, but they have a limited feature set and lack the durability and consistency of the R2. Other units may be specialized for specific types of use.
Weird that they would call this new droid a C* since it appears to be quite clearly an R* unit , and there have been several C* units in the prior star wars universe that are clearly not astromech droids.
But hey...they need to give it some sort of name to appeal to non-nerds...