Each of many different human hormones can by itself raise the concentration of glucose in the blood. The reason for this is probably a metabolic quirk of the brain. To see this, consider that although most human cells can produce energy from fats and proteins, brain cells can use only glucose. Thus, if blood glucose levels fall too low, brain cells will rapidly starve, leading to unconsciousness and death.
The main conclusion of the argument above is that the reason that many different hormones can each independently raise blood glucose levels is probably a metabolic quirk of the brain.