NY laid this down on his screen :
What was the main reason that cars changed from sealed beam to replaceable
bulb in fixed reflector?

I think happened, when halogen lamps came along.

It began with ordinary bulbs, then along came sealed beams - which
improved massively on separate bulbs, then halogen bulbs (brighter,
whiter) demanded separate reflectors and lens again. They couldn't make
halogen work in a sealed beam unit.