CephFS simply didn't gain as much traction because it made sense to just store objects in many cases, and let something else worry about what is stored and where. A massive distributed file system is not nearly as necessary as people make it out to be for a lot of different workloads.
RBD works so well because it can replace iSCSI/FC storage and provide failover and redundancy, and as you scale servers it gets faster. We had 20 Gbit/sec bandwidth for our storage network (to each hypervisor), with 20 Gbit/sec for the front-end Ceph access network, and Ceph RBD easily saturated it on the hypervisor side. Spinning up VM's in OpenStack was a pleasure and soooooo quick (RBD backed images too).
We looked at increasing the hypervisor to 40 Gbit/sec, but decided against it for cost reasons, and in reality we had very little workloads that actually needed that much storage IO.