Something that's disappointing isn't what you'd hoped for. A disappointing baseball season usually means that your favorite team lost a lot of games.
When the candidate you voted for loses, it's a disappointing election result, and a disappointing grade in your math class is lower than what you hoped or expected it would be. Disappointing comes from the verb disappoint, which means "frustrate expectations," but had an earlier definition of "remove from appointed office." The modern sense of disappoint and disappointing comes from "fail to keep an appointment."