I think another reason they prefer JW over non-JW lawyers is because they can pressure/intimidate them to do things that a worldly lawyer would refuse to do for ethical or legal reasons. A loyal JW knows that Ceasars laws should never trump what Jehover wants. Plus they probably don't want non-JWs getting such an up close view of their shenanigans. Wouldn't want to bring reproach on the org.
No Apologies