To understand why, you have to understand the nature of vitamins and phytonutrients (plant-based nutrients). These are very delicate molecules which are fragile. They are easily destroyed by heat, which is why raw plant foods contain more plant nutrients than cooked foods. Carotenoids, antioxidants and other molecules like DIM (in broccoli) or anthocyanins (in purple corn) are all easily destroyed by microwave energy.
Microwaves "nuke" your food at a cellular level
Microwaving is the worst way to cook foods because microwaves excite the water molecules inside whatever you're cooking, causing heat to be formed from the inside out. This results in a cell-by-cell "nuking" of the food (such as broccoli, carrots, etc.), causing the near-total molecular decomposition of the vitamins and phytonutrients that promote disease.
Microwaved food is not merely "dead" food at every level, it is food that has been molecularly deconstructed, leaving nothing but empty calories, fiber and minerals. Virtually the entire vitamin and phytonutrient content has been destroyed.
Anybody who says microwaving food is a healthy way to cook is wildly ignorant of nutrition and cooking methods. While steaming vegetables is, indeed, a good method for some veggies to help make certain nutrients more bioavailable, microwaving destroys veggies from the inside out at a cellular level.
Don't be fooled by appearances
What's really deceptive about microwave cooking is that the food still appears to be basically the same, but at the cellular level, it's like a nuclear war has taken place. The actual molecular structure has been decimated. If you could see microwaved foods with a powerful microscope, you'd never eat them again because you would recognize just what a nutritional wasteland they really are.
If you want to eat in a healthy way, make sure a significant part of your diet is raw plants. It's okay to eat some steamed, stir-fried or even grilled veggies, but when grilling foods, avoid burning them because all burned parts contain toxic, cancer-causing chemicals that promote colon cancer.
Personally, I am absolutely astonished that more people don't already know all this. I'm even having to talk with my own staff about these issues, as they don't seem to know the full story on microwave cooking. Spread the word: Microwaves promote death because they turn nutritious, healthy food into empty, decimated calories. I gave up using a microwave well over a decade ago, and I don't even own one.
Remember, a person who cooks a lot of their food with microwaves will inevitably have chronic nutritional deficiencies that promote cancer, diabetes, bone diseases, loss of brain function, heart disease and many other health problems. The least healthy people of any society are typically the ones who frequently use the microwave oven to "nuke" their foods.