No reason was given for her absence but she has said that Trump's 2017 inauguration was not an enjoyable experience for her.

Obama didn’t say why she’s skipping Trump’s inauguration. But there’s ample evidence that she just has no interest in hiding her feelings.

Michelle Obama is skipping Trump’s inauguration, but former Presidents Obama, Clinton, and Bush will be there.Let’s be real—her absence speaks louder than words. This isn’t just a scheduling conflict; it’s a statement. Sometimes, silence says it all. pic.twitter.com/LjhcyPxjjr