My guess at answering the question, based on some observations during frequent NYC stays.

NYC hotels attract business and expense account travelers.
Many of these people have "status"
Sometimes status get them a late check-out (whether in program terms and conditions explicitly or not)
They use it.
Who doesn't want maximum time in NYC (provided no flight to catch)? And who wants to pay hotels to store bags ( an increasingly common phenomenon)?
Ergo, a hotel full of rooms, empty other than bags, until 4:00PM.