The answer is simple — architecture. Redis originally was designed to replace a SQL database with a single server, in-memory, data structure store to cache data generated by a local website. Its single process, single threaded design focused on combining data structure operations with an in-memory cache. However, the key features required by a distributed cache had to be bolted on later by third parties.
In contrast, ScaleOut StateServer was conceived as a scalable, distributed, in-memory data store with integrated data consistency, sharding, scaling, and high availability. This led to a robust, highly reliable architecture that incorporates the latest industry-leading technology and minimizes management costs.