It could be in part due to training.

Reyna & Brainerd's Fuzzy-trace Theory research suggested that the conjunction fallacy gets more common with age.

And given that some of my guess-the-teacher's-password heuristics more or less directly invoke conjunction fallacy (Given A, B, C, and D, where D is A+B, D is more likely correct than either A or B), I'm inclined to suspect that education might be strongly reinforcing the bias.