If true, my guess: little need for the feature, and an understandable reluctance to make the fuze any more complicated than necessary, especially as failures or inadequacies in various sorts of fuze or triggering device have repeatedly caused serious problems with naval ordnance. See: early-war American WWII torpedos basically just not working, British AP shells in the WWI Battle of Jutland detonating too early and so being far less effective than they should have been.