[michaelt]

  So I don't think we have answer yet
  as to why you wouldn't use USB.

The short answer is "USB has a widely known bad reputation"

How did that bad reputation arise? A combination of factors:

* USB 2.0

* Platforms like Raspberry Pi, where both disk and ethernet are on the same USB link.

* Sellers marketing products with the USB link bitrate (5 Gbit/s) rather than the rate of data transfer from the ideal disk.

* Embedded platforms that don't have the performance needed to saturate a gigabit ethernet link (or 5 gigabit USB 3 link)

* Articles like this, reporting "60-113mbps" without going into enough detail to determine which components are the limiting factors on performance.