[bombcar]

It’s cheaper (or was) - early cars had one red light on each side and could blink it for a turn signal, but it was also the brake.

Later a second light was added for the taillight but they did it in the same bulb.

[darkwater]

Thanks for the explanation! It still seems odd to me that it hasn't been regulated somehow, having 2 different colors for 2 different actions is safer.