IMO, because you can use a RDB as a K/V store, but you can't use a K/V store as a RDB. And I have yet to personally work at a scale where a sharded RDB was incapable of handling the load.

The big relational DBs scale well (near-terabyte with thousands of queries per second) with little maintenance. They scale ridiculously well (terabyte+ with a million+ queries per second) with more maintenance. Most loads simply don't require the scale at which RDBs start to actually falter at.