Quite simply.....Once upon a time at an airport not far away a lowy inexperienced grade 3 instructor who thought that they were an undiscovered test pilot came up with this "rule" and imparted their wisdom on a hapless student who didn't know any better, who(m) in turn became a low experience instructor armed with this superior knowledge who trained somebody, who became a CFI, who then made it law.
Probably the same instructor who advocated reducing the rpm to minimum on both engines before selecting gear down at max limit gear speed, because the slipstream over the gear doors caused by the propellers at cruise RPM would increase the local airflow over the landing gear doors so much that the faster airstream would mean the dynamic pressure would exceed the max gear down IAS and risk damaging the gear mechanism
