If you would like to diagree with the statement, here is a format and ideas:
Introverts are not compelling nor accomplished leaders in the majority of circumstances, because:
1.) A leader should be social and understand what his/her followers need or want. Introverts are people who search inward rather than outward, thus making them seem shut-off or anti-social. A leader who does not have a following, will not gain much ground in accomplishing goals.
2.) Although introverts may be insightful, they are often too shy. This would tie in with #1 because of a lack of proper communication. They would require someone else to speak for them, and this makes a leader appear weak or fearful.
3.) Introverts may be too independent upon themselves. Leaders utilize their dependence AND independence (which can be a trait of extroverts) to access knowledge and have several options rather than simply one or two.