Szczepan Kolaczek

Fellow classic car friends, I am wondering about one thing: in 1912 Charles Kettering built the first automotive electrical system and used 12V. Later on almost all (if not all) American cars had 6V, till mid-50s. Why did they change that? The only reason I can think of is the cheaper batteries, with 3 cells instead of 6. Byt all the wiring needs to be thicker with 6V, the reliability suffers, starter motors are weaker, etc. Can you please explain it to me?