Some of the bigger U.S. County/City government's do not want to deal with the additional maintenance of their helicopters when they get in the 10-15K hour area. So when they buy, they structure the loan they get with that in mind.
Most citizens of the County/City do not want an "old" helicopter above them. They want a "newer" one. It is the same as what the airlines are now going through, in a way.
L.A. County sold theirs as the 520's were, well 520's. The 600's were even worse. Not to mention product support for them. And they needed a bigger helicopter. They outgrew the 500's with the equipment they had onboard.