Why don't all NYC subway stations have these signs?

Electronic sign at the 51st Street Station.
(photo: TheHopefulTraveler)

It's day eight in my Summer 2012 trip to New York City and this means last minute shopping errands while trying to make the most out of my last full day in the city. This morning I find myself at the 51st Street Station waiting for trains 4, 5 or 6 (Green Line).

I take pause here because of the sign above the platform. It lets waiting passengers know how long till the next train and the number of the train. These types of signs are at every London subway station but are less common in NYC. Maybe NYC is in the process of installing these signs at more stations.

It's funny that with the sign people keep looking at the sign for their wait. At subway stations without this electronic sign, people keep peeking down the tunnel for the light of an oncoming train.