Free parking is something that people love, but the actual cost in major cities would blow a big hole in the hotel's budget. Parking is expensive in downtown NYC, Chicago, London, DC, Seattle, and SF because the available space is limited compared to potential demand. High prices for parking make it easier to find a space and encourage people to consider the economics of using a car.
I travel mainly to Marriott hotels in suburban areas where parking is free. When I have a car and staying in a city or suburban area with little parking, I expect to pay for parking.