Doesn't it make sense though? In 1910, manhattan was expensive but a lot of people still lived/worked there. The apartments would be crazily subdivided so you could fit 10 people into a 3 or even 2 bedroom. Now they are all owned by wallstreet bankers with small families, and they spend much of their time somewhere else (they might not even list their manhattan apartment as their "main domicile" during the census).