Peptic ulcer disease is specifically referring to ulcerations the occur in the mucosal lining of the lower esophagus, stomach and duodenum. They are due to one or several of the following causes: abnormal mucus, parietal cell tumor, decreased or increase gastric acid production, increased serum gastrin, increased serum pepsinogen and/or the presence of Helicobater pylori (usually 60-100% of patients with peptic ulcer disease have this). Stress may also be a factor, however, it has been found that the H. pylori bacteria is the primary culprit in the majority of patients. Once this bacterial infection that only thrives in the stomach and duodenum is treated, the ulcers, generally, are easily brought under control and heal up while the patient is given drugs designed to reduce acid production. So, peptic ulcer disease can be succesfully treated and cured.
Pancreatitis, ulcerative colitis, diverticulitis and Crohn's are primarily chronic diseases that do not respond as successfully to treatment as peptic ulcers do. I do not think the H. pylori bacteria, the primary culprit in peptic ulcers, has ever been found in those other structures. Pancreatitis is not as common as peptic ulcer disease although it can exist along with it. Pancreatitis occurs most often in people who have other chronic problems such as alcoholism, drug addiction, and gallbladder problems. While ulcerative colitis also causes ulceration and abscesses to form, the mechanism is different from what is going on in the stomach. Crohn's is much like ulcerative colitis except that it occurs in both the large and small intestine and forms fissures and fistulas as well. Again, the causative mechanism is different from peptic ulcers. Diverticulitis is confined to the colon, primarily the lower segments and unlike Crohn's does not spread into surrounding structures. There are elements to the development of ulcerative colitis and Crohn's that are still a puzzle to the medical profession. Except for peptic ulcer disease, the rest that you mention are progressive destructive diseases that are very seldom cured. They may be put under control, but not cured. Whereas, in the case of peptic ulcer disease, the cause is pretty clear and it can be healed successfully with correct treatment. The same cannot be said of the other diseases you mention.