Delta's current fleet problems are due to a shortage of parts for the PW2000 engines on their 757s. There are presently 14 757s grounded systemwide because their engines have reached the maximum permitted number of cycles before needing an overhaul, but Pratt & Whitney is severely backed up in shipping parts to them.

Obviously in this case, DL's top priority is to service the flights with the most customers. Cancellations are made in markets like PHL because reaccomodation is easy with hourly service.

Hopefully this shortage will be taken care of before the end of the summer and the schedules will be back to normal soon enough.