It probably started during the "bike boom" of the 1970s. During the 60s and 70s, a lot of casual cyclists rode Schwinns and Raleighs that were single speed or had 3-speed Sturmy Archer shifters. Higher end racing bikes of that era had 2x5 gear trains, or 10 speeds total. The name just stuck because so many bikes were sold during that period, even after manufacturers started selling 12 and 14 speed bikes (or triples with 15, 18 or 21 speeds). What's funny is that so many people still refer to high-end road bikes as 10-speeds, even though they quit making bikes with 10 speeds several decades ago.