Earlier quoted context omitted.
> People wonder "why is the subway so filthy?" Because we run our system 24/7. We can’t flood our stations with bleach like they do in D.C.
From the NYT article comments: "In terms of cleanliness, which is also an issue of public health - why doesn't MTA have a dedicated crew of 4 or 5 people with a power washer, soap and squeegees to move methodically through each station, working on the weekends or overnight? " Can't have it, because the MTA is not in the moving-people-business, they are in the wealth-transfer-business.
The stations don't stay clean because they move millions of people per day, not all of whom are clean. Today I saw a kid throwing up on her way into the station; she and her mother were somehow polite enough to pause above a drain to do so. Combine that with people tracking upstairs dirt in, and littering, intentional and accidental (I dropped a candy wrapper while getting my kid home; I'm not stopping to pick it up mid-tantrum), and you've got a pretty messy situation.