Good and Bad business decisions by gun companies:

[Nyles]

Actually, it's not a question of conversion, you literally need to build the gun from scratch. And a Luger is not a simple gun to build, there are a lot of complicated machining operations and tight tolerances involved. The .45 Lugers had literally no parts in common with the 9mm / 7.65mm guns. Everything had to be scaled up. Lugers are big, bulky guns, but all that space is being used, they're quite tight internally.