Shyness and introversion are not the same thing. Shy people fear negative judgment, while introverts simply prefer less stimulation; shyness is inherently painful, and introversion is not. But in a society that prizes the bold and the outspoken, both are perceived as disadvantages.
I work in sales. Our division has about 70 people. We were at a conference last year and it was all about the various "social styles": analytical, driver, amicable, extrovert etc. We had to take a questionnaire and at the end were placed in a social "quadrant". They had all of the "extroverts" stand against one wall and then all of the introverts against another. Sales people are overwhelmingly extroverted. There were only about 9 people against the introvert wall. The extrovert wall was loud, gregarious and in a constant state of motion, while the other wall was calmer, quiet and just sort of soaking it all in. Of the people against the extrovert wall roughly 10% were in a leadership position. Against the introvert wall I'd say 5 out of the 9 were leaders. -Interesting.