The stamps are backed (redeemable) by something real and fixed: shipping a first class letter. So they are more legitimate money (and not just currency) than dollars.
This implies you have more trust in the U.S. Post Office continuity of existence and function than the "full faith and credit of the United States."
Just going by track record. The dollar has been reliable debased every year. Naturally if you have a time frame of decades, stamps would be ill-advised.