Early computers of the 'all in one' form factors, such as the Commodore PET, the early IBM microcomputers and later models of the TRS-80, as well as the 'box' form factors such as the Altair, used a metal case; this continued through the IBM PC and all the clones thereof to the present day.
On the other hand, most early computers of the 'CPU in keyboard' form factor, such as the Apple II, Sinclair Spectrum and Commodore machines from the Vic-20 on, used plastic cases. (The one exception I can recall being the Atari 800.)
A metal case is more robust, conducts heat better and acts as an RF shield. I'm given to understand it is also slightly cheaper. (I've seen a figure of $300,000 quoted for the injection molds for a plastic case in the late seventies, and that was a time when interest rates were much higher than they are today.)
Given all those advantages of a metal case, why did so many computers use a plastic case? I could put Apple down to Steve Jobs having strong aesthetic tastes that greatly differed from mine, but Jack Tramiel and Clive Sinclair were pragmatists; I would expect them to go with a plastic case only if it had cost or other practical advantages.