For a very simple reason. Hitlers cyanide capsules were supplied by the SS. Shortly before his suicide, Hitler received word that Heinrich Himmler, head of the SS, tried to negotiate a (for him) favorable compromise with the Allies. Hitler felt betrayed, issued orders to find and execute Himmler and didn't trust anything that came from the SS from that point on. Therefore, as Raydan already stated, the poisin was to be tested on the dog first. To ensure his own death, Hitler didn't only take the capsule but also shot himself (as well as his wife) the very moment he bit through the glass.