I don't think it's a matter of contracts or allegiance. It's the much simpler fact that money keeps flowing.
If I break a promise to the people who elected me, I won't see the consequences until the next electoral cycle, at which point they probably forgot, and even if not the alternative is voting for the other guy who has also broken campaign promises. There is no reliable retribution. In a world where campaign promises were a sacred oath and breaking them was anomalous, voter punishment might make a difference, but we don't live there.
A campaign donor, on the other hand, can give me money whenever they want, and thus also stop giving me money whenever they want. Even if the other guy also breaks their promises to donors, they can find someone else to prop up, or simply save their money and spend it in yachts or whatever. I will feel the consequences right away.
Reinforcement is much more effective when it's immediate rather than delayed.