It's a bit of a self-fulfilling prophecy -- the more flash chips you've got the more bandwidth is possible, but there's not much incentive to go faster than x4 can if you know the device is going to plug into x4.

If you're building a storage server using PCIe SSDs, you really don't want to have to pay Avago/Broadcom's prices for PCIe switches. That's one of the things that makes AMD's new server platform with 128 PCIe lanes so appealing: you can put together a 2U 24x NVMe SSD server plus NICs without the expense of a PLX chip.

It seems like we'll be sticking with PCIe x4 interfaces for SSDs for quite a while; client/consumer use cases don't allow for higher pin counts, and at the moment there's not much reason to prefer using half as many SSDs with twice the lane count for enterprise storage, especially when it means none of the parts can be shared with the client/consumer ecosystem.