Yes, ignorance is the root cause (well.. technically speaking there is never a "root cause" since you can always ask another round of whys). The problem is that smart and well-educated people can become ignorant.
The studies of these people also found that they are typically big believers in conspiracy theories. And I think that's the crux.
Conspiracy theory is like a cancer of the mind. When you believe in some conspiracy theory, then everything that other people are saying, or evidence they provide, is categorized as invalid, with reasoning that either those people are part of conspiracy itself or hapless masses brainwashed by the conspirators. So effectively, you decided to discount all external inputs on the matter (in other words, you chose to be ignorant).
The ingenious part is that it's not strictly irrational. Even if you close your mind to external evidence, rationality can have its field day, you can still work things out through logic. (And for sure, external inputs can be confusing and look inconsistent too.) It's like in a logic system which is inconsistent - anything can be proven within it, and that limits seeing its own inconsistency. (Luckily, the diversity of possible opinions of conspiracy theorists significantly limits spread of their theories.)
I have seen very smart and educated people fall prey on this. Libertarians, AGW deniers.. They tend to argue on rational/logic grounds rather than look out for the evidence. I think we should teach people, if your theory requires conspiracy of more than, say, 10k people, then it's a giant red flag and you should by all means reconsider the evidence.