Deb...Cause of death is not always documented when euthanasia is performed. A lot of times, clients come in to euthanize their pets, and a lot of the time it's a new client or one that hasn't been seen in a long time. If the animal is suffering, most vets will immediately euthanize to put the animal out of it's misery. A necropsy is extremely helpful if the owner allows the vet to perform one (kudos to the OP for going through with that!) but it doesn't always help with definatively diagnosing a "cause of death"
Without following and examining the patient on a regular basis from the beginning to the end of said "cause" it's next to impossible for a vet to truly diagnose. A veterinarian SHOULD not document a COD without knowing 100% sure that's what it is.
IBD has a lot of "grey areas" and misconceptions about the disease are common...I would compleatly understand if a COD is not documented on this particular animals medical record.