Cancelling last minute once is life. Cancelling as a habit is a message.

Anyone can have a week where plans collapse, and a single cancelled evening means nothing at all. A pattern means something else. Once it becomes what you usually do, the other person stops reading your reasons and starts reading the shape, and the shape says they are the thing you move when something better appears. Most people never say this out loud. They just stop asking.

If there is somebody you keep bailing on, either look at why or stop agreeing in the first place. A no now is much kinder than a yes you already know you will take back.