i think the main reason is that the galvanize on the pipe is not that uniform in its thickness and could create a holding issue with the clamp plates.
another reason not to use galvinzed pipe is that the clamp platess can cause the galvanize to flake off and cause a safty situiation. (cuts from galvanize flakes)
there is cost issue also. (black cheaper)
3/4" dia. pipe for over 24" long clamping seems to work better.(less pipe deflection)
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