I didn’t get that either, but its because the SCU or DIU has to be the first on the FCINET ribbon cable due to it having RS485 bus termination resistors. There is a jumper to disable them and thus theoretically you could use external ones, but I bet FCI figured it was easier to have it be at the top for consistency.
The KDU can actually be mounted ANYWHERE. You could mount it remotely if you put a PBA in the cabinet with the SCU instead. If its in the cabinet, it has to be below the SCU just like everything else, as seen here:
In fact, its optional! Kind of. You need one if you have an addressable SLC unit for device names , but the 7200 can also be all conventional, and thus doesn’t need it! It looks really interesting, as in this reddit post and this ad from when it was first released in 1993:
There could even be multiple in the same place, but no reason to do that unless you’re testing them.