Chainstay mounted RDs predate dropout mounted ones by a few decades. It should also be noted that forward opening dropouts are also a fairly modern design. Earlier bikes had mostly rear opening dropouts of the type now used almost exclusively for single speed bikes.

Schwinn probably went the chainstay route to accommodate wider gearing than the rear mount RDs of the time could handle. Or, in typical Schwinn fashion, just to be different and have something to market.

BTW - if you're interested in derailleur history, read Frank Berto's "the Dancing Chain". It's a great illustrated history of bicycle gearing.