Microwave is a form of radiation and radiation breaks down chemical bonds and causes other unwanted effects. Microwaves don't make nitrates disappear, the radiation simply causes the bacon to heat up. The hot bacon then evaporates the water content. The nitrates are still in the bacon, but they are now dissolved in the water. This makes the bacon more unlikely to rot and spoil. This is useful if you want to keep the bacon for a long time, but it's not good for your health as the nitrates are still present.