[emeritus]

Probably down to manufacturing cost. A two-part lamp with a detachable, customer-replaceable, light-emitting element would need a connector that would increase its cost. I remember when I was with Plessey that you could get a complete Shure cartridge for your Garrard deck from the staff sales shop for considerably less than the cost of a replacement stylus from a record shop. By the time you have factored in the middleman's costs, there would probably be little retail price difference between a one part lamp and half of a two-part lamp.