Roman and Greek clothes

I think the trend originally began in the various pulp magazines and serials that were common around that time. They often featured sci-fi stories with decadent and despotic space empires, so a Roman/Greek motif may have seemed like a natural fit. Over time I imagine people just began to associate the style of clothing with futuristic settings in general, kind of like how people in the 50's/60's associated futuristic settings with chrome appliances and robot servants.