My hamster died but why?

It sounds to me like the poor little thing was suffering pretty bad in its last days. The weakness could have been a symptom of anemia or something of that sort. Possibly some sickness brought on by the mites which may or may not have been there before it died. Most likely it died from being unable to breathe. Likely the struggle to breathe was too much for it's little lungs and diaphragm. I'd say it may just have to remain a mystery and you can take comfort in the fact that she's not suffering anymore. Sorry if this is no help at all. This is my first attempt at answering a question on here.