In a nutshell, it is for safety reasons. Without precautions, microwave cozies can otherwise catch fire. This is why my cozies are always made using 100% cotton fabric AND thread. The inside batting is specialized, designed especially for use in the microwave oven. I do not use anything else.
Microwave products require special attention when washing because of any residue left by the washing machine, such as fragrances found in washing machine bead additives and anti-wrinkle chemicals that are found in dryer sheets.
If your microwave items require cleaning, it is best to wash with only a little detergent. DO NOT use fragrant enhancing bead additives or fabric softener. Just as importantly, DO NOT use dryer sheets when drying.