An offer is NOT a type of promise. An offer is merely a communication of the type of promise you might be willing to be bound by. You can use promissory estoppel when someone breaks a promise, but you cannot use it when someone revokes an offer. In a bilateral contract, once the offer has been accepted, it becomes a promise.
When you make an offer, it is true that the offeree has the power to hold you to that promise (by accepting it), but it's not a promise to enter into a contract because you can revoke it at any time.