Because he was afraid laypersons would not understand that E is for Electromotive force (Volts) and though that V for volts would be better understood, but on the other hand, the letter I for Amperes (amps) might not be too well understood by people not exposed to Ohm's law either

Some new descriptions of ohms law show it as V instead of the E.
E= Electromotive force measured in Volts
I= Current measured in Amperes (amps)
R= Resistance measured in Ohms
P= Power, measured in watts.
E=IR Ohms law... not just a good idea... its the law.! Using 9th grade algebra we can solve for any remaining variable if we know 2 of them.
P=EI Power formula. at least in DC circuits.