It’s probably because the subway still uses 1930’s technology to keep track of trains: cables and mechanical switches.
Computerizing the train control system will allow more trains to run more quickly but the MTA is in no hurry. The project started in 1991 and has upgraded only the L train. The target date to finish the rest of the subway is 2045 and that’s optimistic.
No one wants their subway line closed for long stretches of time and since the subway is supposed to run 24/7, that doesn’t give maintenance workers much time to do the work. As we’ve mentioned before, the MTA is perennially short of funding, making it difficult to fund important repairs like this. It also doesn’t help that Gov. Andrew Cuomo and the legislature cut $400 million from the MTA’s budget for the project.