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The health and safety aspect is mind-numbingly overbearing. And for good reason. It is an underground workspace that is surrounded by the general public. When escalators are replaced the entire work area gets boxed in. There must be significant controls in terms of dust, gas (like welding equipment), noise etc. Fire is a significant risk and the underground has history that makes them anxious about fire and escalators (see Kings Cross Fire). Most of the work probably has to take place after hours, and subject to other maintenance taking place. On the underground, you can't just put a new escalator in a docker container and spin it up. The Underground is a significantly complex environment.

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.

Probably because they're bespoke, subject to heavier use, located in confined spaces, and are expected to run 20 hours a day for 50 years.

Reminds me of this interesting behind-the-scenes look at an escalator replacement:

https://news.ycombinator.com/item?id=28207386