Only God and Tullio know (and that's not to infer they're the same person). Seriously, it appears to have been related to the Italian threading, as the steerer did not come in any other thread standards. The steerer was offered during the boom, when Reynolds was considered the best tubeset by North Americans. Many Italian builders used Reynolds on their top frames to ensure penetration of the profitable North American market, but Reynolds steerers were only offered in French and English threading at the time, so I assume it was to allow Italian builders the option of using Reynolds or other non-Italian tubesets, without the extra expense of buying non-threaded columns and having to thread them (those Campagnolo dies aren't cheap).