And here we are approaching the crux of the issue. Building a new line so it's fully automatic is relatively straightforward. Re-engineering an existing line to make it automatic (without shutting it down for months, which understandably is rarely an option) is like performing open heart surgery on a patient while he is running a marathon.
Well, and the SSR is particularly hard (given National Rail services run on parts of the LU-owned SSR infrastructure); the Northern Line automation was nowhere near as painful.
Less than what's happened with the London Underground Sub-Surface Railway, which is now on the third resignalling contract to get them to GoA Level 3.