If it has to be answered in one word, its “performance”. Aerospike’s performance is much better than any clustered-nosql solutions out there. Higher performance per-node means smaller cluster which is lower TCO (Total Cost of Ownership) and maintenance. Aerospike does auto-clustering, auto-sharding, auto-rebalancing (when cluster state changes) most of which needs manual steps in other databases.
I said “clustered” because I dont want to mix redis in that group (though redis clustering is in beta). Pure in-memory performance of Aerospike and redis will be comparable. But Redis expects a lot of things to be handled at the application layer like sharding, request redirection etc. Even though redis has a way to persist (snapshot or AOF), it has its own problems as its designed more like an addon. Aerospike is developed natively with persistence in mind. The clustering of redis also involves setting up master slave etc. You may want to take a look at this talk comparing and contrasting redis vs aerospike.