When you fail to uphold a promise, you have betrayed an individuals trust.
Dishonesty is, by definition, betrayal.
Besides, it is a semantics argument.
Betrayal happens anytime an individual knowingly lies about something, it is even more prominent when the dishonesty is perpetrated against someone who trusted the betrayer.
This is why the phrase, "betrayed my trust" exists.
Breaking a promise is bad mind you, but it's less severe than betrayal, but a long shot. Betrayal is pretending to be a friend and stabbing them. Not keeping a promise is usually never deliberate. Things happen. No one is perfect. We all disappoint once in a while. So while disappointment is not good, it's not like betrayal.