When they first came out, microwave ovens were supposed to be the be-all, end-all. Remember the Amana Radarange? About the size of a small dog house? Took a crane to install it?
It didn't take long for people to figure out that microwave ovens were good for some things (like warming up food) and not good for other things (like roasting a turkey).
That's why they got smaller. No more need for a crane. Or an extra room to house the damned thing.
Actually no.....the original microwave ovens were constructed in that manner for shielding purposes, to keep the magnetron tube from slinging M/W radiation all over your kitchen, and roasting everything within ten feet of it.
They became much smaller and lighter when a very bright guy at Bell Labs figured out a way to impregnate plastic (polycarbonate) sheets with ferric/cupric oxide particles, so that the shielding could be accomplished with much lighter materials, and significantly less of them. They relied initially on a double steel shell (inner and outer) .
Commercial microwave ovens in the 2500 - 3500 watt range even today are constructed with steel shielding similar to the original Amana.
The Radarange microwave had an output of about 600 watts, and it is not uncommon to find them in the 1200 - 1800 watt range now........so the cooking capability has actually increased, as the size and weight became smaller.
It is true, however, that they were launched with great hype, that far exceeded their practical applications. You are correct that they do some things very well, and others, not so well. We had one of the first, and actually tried to bake pastry, and roast a chicken in it.........although the chicken WAS cooked, it was not very appetising in appearance. It was great for baking potatoes quickly though, once you figured out how to prevent them from exploding while cooking (same with eggs and hot dogs).......
doc