Prior to charging for drinks we had service guidelines with every city pair listed, and the services ranged from "request only", to express service, to full service. Once we started charging every flight was a full service because initially we were serving a lot less and the service could be accomplished in a fairly short time frame. Now that we're back to complimentary drinks they have once again put different service levels into place. The service offered is not based on flying time, it is based on NM (nautical miles). Several city pairs are padded (another topic altogether) so what might look like a longer flight in reality might actually be very short. With any flight you have to automatically subtract a minimum of 20 minutes from the flight time; 10 minutes after take off to reach a point where it's safe to take the carts out and set them up and 10 minutes before landing to break the carts down, pick up refuse and secure the galleys and cabin for landing.
PHL-PIT used to be an express flight, meaning the offering on those flights was a tray of coffee, orange juice and water in the morning and soft drinks and water in the afternoon/evening. If it's still an express offering I'm not sure why they didn't offer it. Was the flight turbulent? Do you know if the cockpit crew advised them to stay seated because of anticipated turbulence or an unusually high tail wind making the flight even quicker than normal? Or chances are the f/a's WERE just lazy.
I'm sorry you were disappointed, but in the future if you're thirsty please don't hesitate to ask for something. That's why they call them "request only" flights, so that people who are very thirsty can get something to drink.
Fly safe