In Real Life: I see it partly as a function of the fact that there is only so much time (so developing one area very, very much can take away from developing in another), partly because people who are strong in one area but not others seem naturally to outnumber the Renaissance people, and partly because developing a lot of strength in one area can be a response to being weak in others. Then, once there are a number of lopsidedly-developed people in a job or social area, others have less pressure to be well rounded instead of lopsided.

In Stereotypes: I think a lot of these stereotypes are based int he idea that life is fair, so being gifted in one area means being shorted in another. The flip side of the sterotype that blondes are dumb.