That's a good question; so why DON'T we let go of negative beliefs? Surely if a belief is negative and holding us back, we'd want to let it go; I know I would. The "trick" is realising that our subconscious is holding on to it because in some way it is serving us, and quite often that means it is keeping us safe. Maybe in the past we've been hurt in one way or another and by not allowing the situation to happen again, we won't be hurt again.
So you need to identify the "positive" thing you are learning from this "negative" situation.
When a person has been through a traumatic situation it can be a bit of a "bitter pill to swallow" to ask them what they are learning from the situation, or why they are choosing to hold on to it, but once they do acknowledge it they can move forward.
This is also why you cannot just pull all negative beliefs - your subconscious doesn't hold on to negative beliefs only positive ones. In ThetaHealing we use a technique called digging to identify a person's beliefs - and they're quite often something quite unexpected - otherwise they'd probably have figured it out before!