Well, I can guess at a few reasons. 1. As mentioned elsewhere, the radio signal path loss through salt water is astoundingly high -- I did the math once. 2. WW II era radio technology was bulky, and did not work well on batteries -- vacuum tube filaments eat a lot of power and the plate supply requires a high voltage battery. 3. WW II era radio technology did not have encryption built-in -- it would have been pretty easy for the enemy to signal the torpedo to blow up before you launched it, which would kind of ruin your afternoon.