I guess there's also a democratic argument here - the judicial branch is not accountable to the people directly but merely to the laws stemming from the people, so a deontological approach to upholding these laws is basically the basis of their democratic legitimacy. When they start making consequentialist or utilitarianist arguments it basically means they're engaged in judicial activism, which is often seen as a bad thing - that's not what the role of judges is traditionally supposed to be.
For the other branches it's much more complicated, as they're supposed to represent the people more directly. They don't choose their moral code - the public does when it votes for them.
I just got home, it's Friday night here and I'm a little drunk, so I don't know if that makes sense haha. It's an interesting question.