I believe the reason this isn't taught is because the gun is a much smaller agile target to go for in this sort of situation. It requires a lot more skill and training to accurately remove a gun in that scenario. IF you have the training to do so, then it is definitely the better strategy to go for, but these public safety programs aren't geared towards that approach. They're geared towards the average office worker or student, which just simply can't be expected to do so reliably. 

I don't think you *shouldn't* aim to disable the gun as a priority, but disabling the person is just much more straightforwards and easy to teach to the everyman.