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.
The chapter on judgment under uncertainty in the (excellent) new Oxford Handbook of Cognitive Psychology has a handy little section on recent critiques of the "heuristics and biases" tradition. It also discusses problems with the somewhat-competing "fast and frugal heuristics" school of thought, but for now let me just quote the section on heuristics and biases (pp. 608-609):
My thanks to MIRI intern Stephen Barnes for transcribing this text.