I cannot speak for a employers, but I would say they've found a better person for the position. Either it be by age, sex, religion, health, ect... They're in the Business to make money and they've chosen somebody who is younger, more eager for the position than you. Accept it and move on.
Younger. More eager for the position? I doubt it. In fact I have moved on again and again, like so many here. You are speaking directly to discrimination that is illegal in the United States. We have a formal complaint process. I've never filed one. I have had a clear opportunity many many times. I'm sure many of us here have. We may need to start organizing. We may need to start filing these with enthusiasm and pride. This is how other discrimination battles are fought. People are kept silent on this one for fear of never working again. Perhaps it is time to stand up.