One assumes he didn't want to fire Hakstol. And, as ever with these sort of things, he also wasn't hired by his current boss (in the Flyers case it is essentially like they have a different owner, even).
(Or rather, Holmgren has a different boss. Back to the future?)
“We thank Ron for his many significant contributions, but it has become clear that we no longer share the same philosophical approach concerning the direction of the team,” team president Paul Holmgren said in a press release. “In light of these differences, we feel it’s in the organization's best interests to make a change, effective immediately. I have already begun a process to identify and select our next general manager, which we hope to complete as soon as possible."