The pancreas has to produce just the right amount of insulin at all times. To much blood sugar drops and death occurs. To little and blood sugar rises and death can occur. Anyone who has diabetes can tell you how vital it is to keep blood sugar under control.
Improper blood sugar also leads to secondary problems. Circulation problems, wound healing problems, kidney problems, and eye damage. How would evolution deal with being able to evolve a system that would tell the pancreas what to excrete, when to excrete it, and exactly how much to excrete?