Once upon a time in the Pleistocene...
Before the 1970's, there was basically no market for adult bikes. 26" = ETRTO 559 was the cruiser bike size for teenagers and beach resorts. These cruiser bikes were used as the basis for the earliest mountain bikes, which had a boom in the 80's, and proliferated.
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Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."