Originally Posted by FlyBitcoin

But more work cleaning up since they will have to collect both bulky cans and cups for the final trash service instead of just cups.
Not giving cans allows them to dispose of the empty cans sooner, beginning when the drink cart has completed its rounds.

My guess is that the system engineering probably works in favor of giving cans (even without request) for longer flights that don't normally have a second beverage service, but not for flights with two beverage services or 90 minutes or less.

The FA's at AirTran always asked if we wanted the can before they either located the already opened can, or just grabbed a new one.