This might be unfair to deontologists, but I keep getting the feeling that deontology is a kind of "beginner's ethics". In other words, deontology is the kind of ethical system you get once you build it entirely around ethical injunctions, which is entirely reasonable if you don't have the computing power to calculate the probable consequences of your actions with a very high degree of confidence. So you resort to what are basically cached rules that seem to work most of the time, and elevate those to axioms instead of treating them as heuristics.

And before I'm accused of missing the difference between consequentialism and deontology: no, I don't claim that deontologists actually consciously think that this is why they're deontologists. It does, however, seem like a plausible explanation of the (either development psychological or evolutionary) reason why people end up adopting deontology.