From a few cases I have seen, it is partly due to the very long service life. New units are substantially different from older ones, particularly in all the bit

From a few cases I have seen, it is partly due to the very long service life. New units are substantially different from older ones, particularly in all the bits of it that you do not see as a pedestrian. Often times the retrofit involves a lot of concrete work and other work to adapt the mounting and support points to accommodate the newer designs. Couple this with the fact that you are usually doing the work in sub-optimal conditions because they are trying to keep the rest of the area open for pedestrians, so there is additional safety considerations that you don't have in a new construction install.