Charismatic villains have an edge over their fellows. They can cajole and manipulate, making others believe anything they want them to, without having to use any superhuman powers.
Doctor Doom is amazing at many things. He's intelligent, a superhuman engineer, skilled with magic, and a fierce fighter. He's a great leader, as well, having led the people of Latveria in a rebellion that made him their ruler. The people of his homeland love him and they know that he loves them. Doom can be bombastic, angry, and petty, but he can also be charming.
Doom is known for his distinct way of speaking, and there's a nobility surrounding him that can draw people in. While he can turn on a dime from a charming man into an egotistical monster, he's been able to make even his enemies see things his way when he needs to.