Walrus, it was all about the price. With only 2P60 amp mains for the sub-section(s), these panels were cheap. Many of them were sold "naked", meaning that the sub-section main breakers were field-provided and installed. A molded-case 200 amp breaker easily added a hundred bucks to the cost of the panel. In new home construction, these were mighty popular.
They were also very popular in total-electric homes. I'm not so sure as to why, but I guess that the pole spaces in the main section allowed for higher-amperage breakers than most of today's main breaker panels. It isn't uncommon to have up to 100 amp breakers for electric furnaces. My main breaker panel has an 80 amp breaker for the furnace and it just doesn't "look" right for some reason.
