popcorn pops because microwaves heat water very well. as the water heats (and thus expands), it eventually pops the kernel open, and the starch becomes delicious foamy goodness. however, the water is heated even further by this expansion, and the large majority exits the kernel as steam. as a result, the popped popcorn is left very dry.
the problem here is that you have a very low density and very dry object being heated past its cooking point. fluids - especially water - have a very high heat capacity, and also serve to evenly distribute heat throughout the object holding the water content. things like potatoes and chicken don't pop and expel all of their fluid content, which is why they don't burn in a microwave.
