The TL;DR is that the L (and soon 7) line is automated with something called "communication-based train control". They still have an operator and conductor on every train, even though the train is perfectly capable of driving itself and making station announcements itself.
I think the MTA thought they could kill off the conductor's position when the trains were fully automated, because the operator could take on the conductor's role now that they don't have to actually operate the train. The union did not allow that.
In the end, this is not really the MTA's biggest problem. CBTC was supposed to increase capacity on the L, but it didn't in reality because the MTA doesn't own enough rolling stock to actually offer that capacity, and the terminals on the L don't allow them to turn trains quickly enough to make the signalling system the bottleneck. (The 7 will be better, though, as at least the Hudson Yards terminal has tail tracks that allow the trains to enter the terminal at full speed. The same cannot be said of 8th Ave. on the L, where trains have to crawl into the terminal because the track ends a few feet after the end of the platform.)