* I have a small kitchen and just never use it
* I have a small kitchen and it lacks some feature (eg, grill) that would save me another device.
* Grandma can't made head or tails of these weird buttons. Seriously, my current mirowave damn near has hieroglyphs on it, and it's quite recent. Microwave UI is just appalling. Feel free to ask for a picture if curious.
* We remodeled the kitchen and it's no longer a good fit
* We had an extra one for some reason. Eg, child had to move back in with their parents, brought their stuff.
* It's too small
* I'm moving to another country and not taking it with me
As for why not sell it/list it somewhere? You can buy a microwave for $50, so it may just not be worth the effort. You have to do all this negotiating/chatting/texting and agreeing with people about who will be where when, and if you're not in a bad need of a few bucks it may just not be worthwhile.