The basic mistake seems to be loss aversion, the tendency to regret losses more than one values gains.

I tend to think of loss aversion as a preference rather than a mental error, but I agree that it probably explains a lot of the "don't know" answers. What I cannot figure out is why the test designers want an individual's level of loss aversion to affect their score.

Another possible (and probably over-charitable) explanation for the lack of guessing is that students are afraid of being drawn to the wrong answer. For example, I've heard that on the SAT math portion many of the answer choices purposely contain numbers that were mentioned in the question, drawing "random" guessers because such answers look more plausible.