It's because of the '70's bike boom. It was a unique period of time in the US, in that bicycles actually outsold cars for several years.

From Wikipedia:
"At the height of the boom, in 1972, 1973, and 1974, more bicycles than automobiles were sold in the U.S.

Additional factors contributing to the U.S. bike boom included affordable and versatile 10-speed derailleur-geared racing bicycles becoming widely available,[SUP][8][/SUP] the arrival of many post-World War IIbaby boomers at adulthood and demanding inexpensive transportation for recreation and exercise, and increasing interest in reducing pollution."

As noted above, these bikes featured a front double and five speed cassette, thus providing 10 usable gears. The name '10 speed' captured the imagination of the general public and'stuck'.

Keep in mind that during this period of time most adults had only ridden single speed bikes, or 3-speeds via an internally gear hub. Derailer equipped bikes had been available for many years, but they were expensive enough that only racers really bought them. So the idea of having 10 gears seemed amazing.

I got it. I was about to look for a picture to remind the poster of what an ordinary bicycle is.



10 speeds were called 10 speeds because regular american bicycles of the '60s had only one speed, or an imported bike might have 3 speeds.

I remember going from a 3 speed roadster to a 10 speed drop bar in the 70's and feeling like I'd found the turbo charger. It was awesome. The 10 speed was a gift from my Grandfather who preferred the slant 6 in his cars.

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.

Because that is how Schwinn and its dealers described them when they introduced their first 10-speed models in 1960...

Actually metacortex come closest to the right answer. "Ten Speed" basically separated the 10 speed sport bikes from the iron pipe coaster brake models of the time.