Why can't people who have had gallbladder problems use alli?

Risk of gallstones: Rapid weight loss increases the risk of gallstones. This is likely a reason for the recommendation against orlistat (alli or xenical) for weight loss in people with gallstones. There may be other reason also.

Steatorrhea: Alli impairs fat absorption by blocking the pancreatic enzyme lipase. Biliary disease, if there's excess bile in the intestine can cause steatorrhea (fecal loss of bile fats/salts). The increased loose stool production by Alli effects on biliary steatorrhea can quickly cause dehydration, electrolyte disorders, etc... There's no substitute for smart dieting and exercise...