What is the cause of normal sgpt level found in cirrhosis?

Not unusual. It's not unusual to have normal liver enzyme levels when you have cirrhosis. These enzymes are elevated when there's is ongoing liver damage. Cirrhosis is scarring of the liver. If the damage to the liver has already been done, the levels may be normal.
Sobered up? Dying? Especially if the liver damage is intermittent, as in someone who drinks heavily but then goes sober for a while, the SGPT is likely to return to normal during abstinence. In end-stage cirrhosis from any cause, most of the liver cells are gone and there's no longer a source for SGPT. You'll know which by the history.