the only right way to answer that question involves a 10000 word history of the politics of the MTA, the finances of NY state and NY city, bankruptcies, budget

the only right way to answer that question involves a 10000 word history of the politics of the MTA, the finances of NY state and NY city, bankruptcies, budget shortfalls, political corruption and misdirection, very messy labor union politics, and the complexities of running a giant subway system 24/7 in a region that suffers significant damage every year from hurricanes and ice storms.