MTA union, it's an extremely powerful union. The L line for example is set up to be fully automated, but they are still required to have staff for the train.
From what I understand, the L train is one of few that has the necessary upgrades, primarily because it doesn't really intersect with other lines, which makes it a lot easier to deploy. If I recall, the L functions with only one conductor when the other lines use two. I agree that the union could be more progressive and embrace change... but the big problem is money, and consequently, state politics.