Being a little familiar with federal budgeting, the key word in this is "OBLIGATIONS." If FEMA is legally OBLIGED to reserve these funds for previous disasters. then there's likely nothing that can be done short of an act of Congress to de-obligate them. If there IS a process in place, and there's no realistic way those funds will ever be spent, and it's only through laziness/inattention that this money hasn't been reallocated, then that's on FEMA.
The other issue is that there is also a very strong chance that de-obligated funds go back to Congress, NOT stay with FEMA to be used elsewhere, which is also a reason that agencies are reluctant to close out unspent money.