Many downtown's in the USA are made up mostly of office buildings. If you do not show up during work hours of 9-5 Monday- Friday then they will be empty. There are exceptions of course and some downtown's are starting to have more residential built into them, but still, they are mostly dead during non-work hours.
As for the reasons, they are many, white flight of the 60's and 70's which emptied many city centers, also, new zoning laws which made mix used development hard to built, the explosion of suburbs, and the list goes on.
Also, those million plus people cities are spread over an enormous area in almost all US cities, mostly in low density suburbs. This is why even a city with millions will have downtown centers that can be completely empty at night.
As for the reasons, they are many, white flight of the 60's and 70's which emptied many city centers, also, new zoning laws which made mix used development hard to built, the explosion of suburbs, and the list goes on.
Also, those million plus people cities are spread over an enormous area in almost all US cities, mostly in low density suburbs. This is why even a city with millions will have downtown centers that can be completely empty at night.