Why can't doctors do the lapband surgery with only an epidural instead of anesthesia?

Location. The epidural anesthetic would not be able to provide enough anesthesia to the abdomen at the level of the diaphragm. If you doe the epidural too high then the patient loses the ability to breathe properly. You would also be extremely uncomfortable.
Laparoscopy. Requires insufflation of peritoneal cavity with carbon dioxide gas, which requires anesthesia up to top of abdomen. If you give epidural anesthesia to that level, it will also stop your breathing.