Has anyone tried to find out in detail who uses a bike for transportation and why?
First divide the statistical population into those who choose the bike and those who are sentenced to it. Reluctant cyclists who are simply enduring driver's license suspension or revocation may find themselves changed by the experience. These changes may be temporary or permanent. But they became cyclists by accident, so to speak.
Just scanning people I know or have heard of, the exploratory or transporational cyclists seem to lean heavily toward mechanical or engineering types who like the machine as a machine, and various kinds of humanities majors (to use a higher education term), who like the necessary human element a rider brings to the equation.
Some riders combine mechanical aptitude with human romanticism. It's a continuum, with very mechanically inclined riders at one end and mechanically helpless but hopelessly enamored enthusiasts at the other.
Mere educational credentials do not fully indicate intellectual capacity and interest. Since any more or less able-bodied person can hop on a bicycle and make it go, the pool of riders is vast and hard to sort.
I'm going to guess you will find fewer people with business degrees than with liberal arts degrees commuting by bike. Partly that has to do with the preponderance of liberal arts graduates in the fast food industry and other service jobs, compared to people who kept their eyes on the financial prize throughout their academic career.
Hey, prove me wrong. I'm a liberal arts graduate working with dirt, for dirt. You're only worth what you're worth, after all.
There are many practical reasons to use a bike to get around, but none of them counts a damn if you don't like to ride. That is the single unifying characteristic of all riders.