Platform doors are generally installed when you install modern signalling (CBTC) onto a line, which much of the NYC subway does not have. CBTC equipped trains can operate under Automatic Train Operation (ATO) meaning that trains can be made to always stop at the same location on the platform.
Furthermore, platform doors require you to have trains that have the doors in the same positions on the car body, which much of the newer subway fleet (R142,R188 & R143,R160,R179) has but the legacy fleet does not. This issue get compounded even more where multiple lines intersect (which is often due to the NYC subway's unique connectedness) requiring that certain stations would need to support 3 or 4 different car classes.
Furthermore, platform doors require you to have trains that have the doors in the same positions on the car body, which much of the newer subway fleet (R142,R188 & R143,R160,R179) has but the legacy fleet does not. This issue get compounded even more where multiple lines intersect (which is often due to the NYC subway's unique connectedness) requiring that certain stations would need to support 3 or 4 different car classes.