I think cult leaders are charismatic and charming, and that in and of itself is enough to control a lot of people. When you combine this with weak personalities and you have a recipe for disaster--or, well, a cult. But every religion is a cult, really. If Christianity just started, it would be considered a cult.
I believe history has shown us that cult leaders do prey on those with low will power and self confidence. This is the only way these people can gather followers. People who are apt to question things are not going to fall prey to a cult leader. Cult leaders know this and they will avoid those who are going to upset their plans.
Everyone knows that people with low self confidence and low will power are traditionally followers. These are the types that cult leaders prey on because they are easily led and don't usually speak up in their own behalf. They allow other people to coerce them into a lifestyle that stronger individuals would not be taken in by.
Cult leaders do not prey on those with low will power and self confidence. Cults attract people who have the need to be part of something that is a little subversive. This includes people who are very strong-willed as well. If there were only low will-power people in cults, they would have no leaders.