2 Answers

because they don't have the "word of honor".  

A promise is important at its inception. It's a unique relationship between two (sometimes more). It's usually based on closeness and caring. When one cares for someone, there is implicit trust. It's hard to make a promise with someone one doesn't trust from the start.
SOMETIMES, the relationship loses its caring and closeness. The trust becomes unimportant because the care is gone. In that case, the promise loses its importance. "If I don't really care anymore, what's the point of carrying the burden of this promise?"

Someone very dear to me has been through this experience recently. It hurts everybody. Best to be very, very careful about making a promise to anyone. They are difficult to keep and something you should never give away.