Why do some Hilton hotels allow you to pre-pay and others don't?

Been booking a few hotels for a forthcoming holiday and the pre-pay rates, when offered, are generally $20-40 cheaper than the "reserve with you credit card" rate.
I know I want to stay there as everything else is already booked.

So why is it some hotels only offer the reserve a room option?
Is it a Hilton thing Vs. Hamptons etc?