A microwave oven cooks food by bombarding it with energetic waves at a particular frequency. When the microwaves hit water molecules in the food, the molecules heat up, and the food cooks from within.
In order for food to get crisp, though, the exterior of the food needs to get hot enough to evaporate water. As the water is driven off, it leaves behind a crisply porous shell; because water is no longer keeping the temperature below 212 degrees, the surface also starts to undergo the delicious browning reactions that start at around 250 degrees.
That doesn’t happen in the microwave. As water evaporates from the surface, the surface absorbs less energy, not more, and the microwave energy starts to concentrate on the interior of the food. As a result, the interior can scorch while the surface just becomes dry, tough, and pale.