Why isn't Columbus Circle an express stop on the 2 and 3 trains?

So we can zip from 42nd to 72nd in five minutes, of course!

Much of Columbus Circle was compromised when the IND subway was built intersecting it along 8th Avenue in the 1930s. Much of the station was lost so a connection to the newer subway could be made, but ample room was also left because there was a plan to make Columbus Circle an express station and convert 72nd Street to local. The platforms flange out very wide in spots as space was left for this now-scrapped objective.