Popcorn bags tell you not to use the popcorn button on your microwave because, in most cases, the button needs to provide the proper amount of time for making popcorn. Most microwave popcorn buttons typically run the microwave for far too long to cook popcorn. Moreover, because the popcorn option in many microwaves is just a timer that lacks a humidity sensor, the microwave cannot detect when something inside is burning.
In addition, different popcorn varieties or bags have different weights and humidity levels and therefore require different cooking times.
