I agree with 3alarmer. In the '50s/'60s both my father and I had 3-speed bikes. But when the bike boom came around 1970 10-speed bikes were marketed as 'The Bike' to have. I imagine part of it was a desire to capitalize on the bike boom as much as possible. Lots of people already had an old 3-speed sitting neglected in the garage or basement so if they came into a bike shop and saw new 3-speeds they might opt to go home and try riding their old one first. But if they could be convinced that 3-speeds had been made obsolete by the new derailleur bikes then the shop could make a sale.