because back in those days, it was easier to distribute current via multiple 12V rails instead of a single one. SMPS power supplies were a relatively new thing back in those days. Components were not NEARLY asefficient then as they are now. Most SMPS based systems were current-mode based, becuase they used bipolar transistors as chopper transistors. By the time the SE came out though i think MOSFETS that had enough power hit the scene and they just started using them, but the on-resistance was not good. Still inefficient and generated heat, however though still the efficiency in the saturation region on bipolar was horrible. Nowadays high power MOSFET came along.

Today, we have MOSFETS which are voltage-controlled systems, and have saturation on-resistances in the thousandths of milliohms. orders of magnitude more efficient than we had back then.

Thats the only thing I can figure, as back then it was much easier to make multiple rails of smaller amperage, than one large single rail thats high current.