PCIe flash cards are just expensive disk drives
We're seeing the storage industry go full circle with flash.
The reason SAN/NAS was invented in the first place was that disk drives were the most expensive component in a server. By aggregating disks in a centralized location (outside of the server) capacity utilization and availability improved. A PCIe flash card in a server is just like a disk drive in a server, no other server can share it's capacity/performance resources = poor utilization of a very expensive resource; and for 99% of the implementations out there, if the card goes offline, you're data is offline or worse yet, you lose data.
PCIe flash cards still cost about as much as the entire server and are, again, the most expensive component of the server. It's a natural evolution that flash will migrate out of the server and into shared storage environments for economic efficiency.