I think failed is a bit harsh. Guy didn't have much time.
Witnesses say he was struggling to get out. It sounds like his harness quick-release jammed up. That's why I always carry a knife to cut my harness loose (when I am allowed to that is).
Procedures and checklists.... When in the Navy, we would hold ditching drills on a regular basis. There's no substitute for training until following procedure is automatic. It's unfortunate that some warbird pilots don't have the emergency procedures memorized. I've talked to several, who fly WWII aircraft, who stated privately that this is a common issue, especially among guest pilots.
We probably won't know exactly why Gordon did not get out. What we do know is that he did not jettison the canopy, and that may have been a significant contributor.
Other possibilities have been discussed. Had he not locked his harness, etc.....