My guess is that the idea probably came first, as a natural evolution of having roads where vehicle speeds (maybe potential speeds) exceed the capabilities of cyclists by a considerable margin. I think about when I'm in a car going up a steep mountain road and get stuck behind a Winnebago struggling to maintain 15 MPH. I just want that sucker to get out of the way. As luck would have it, many such roads have developed "truck lanes" or turnouts to facilitate getting around. Perhaps bike lanes are an extension of that idea.

Not that I consider BIKE = WINNEBAGO or anything like that, but there is an obvious discrepancy in capability of both "Winnebagos on mountain roads" and "bicycles on roads where speeds exceed 20 MPH" and I could see how people could get the idea that there's a need for separation.

I'm not saying bike lanes are a good idea.

Or a bad one.