My definition of introvert is "drained by other people" (vs. extrovert as "energized by other people"). I'm not shy... I'm outgoing, friendly, good at shooting the shit. But I get home and I'm tired.
Extroverts, by virtue of that energy-boost, are more likely to get into a enjoy sales... and do what it takes to be a good salesperson (research, etc.). So that means that the average introvert in sales is a "better" salesperson than the average extrovert, because the extrovert population is so much larger/deeper. There's no such think as a low-performing introvert in sales, because that introvert wouldn't be in sales in the first place.