I read in a science magazine that things called quasars in outer space emit microwaves, but I held a bag of popcorn up to the sky one night for a whole hour, and none of it popped. But the two things produce very different microwaves. In order to answer to your question, one should consider exactly what microwaves are. Everyone knows what waves are, and every wave has a particular size, called its wavelength.
Corn kernels are small things, too, which is why they get popped by small waves.
However, each kernel has its own unique size, which corresponds to the wavelength that can pop it. The oven then transmits a microwave to pop each kernel. Most of these microwaves aren't the right size to pop your corn. They worked by heating oil and air until they produced lots of microwaves in roughly the right frequencies to pop the corn. Everyone has abandoned those old devices, in exchange for modern, efficient microwave ovens.