Why doesn’t all the popcorn pop when you pop it in the microwave?
All the kernels would pop if you gave them enough time. The trouble is, if you did that, the first ones to pop would be burned. Kernels pop by virtue of their moisture content, their size, and the thickness of their shells. So if you could find a bag in which all the kernels were exactly the same in those ways, all of your popcorn would pop at once.