Whats your definition of a tank? Has a big high velocity gun, tracked, armored, seems like a tank 
Take a look at the lightly armored turret. In fact, the whole thing is lightly armored compared to even a light tank. It is not designed to do the jobs a tank is designed to do. It is 1/2 the weight of a Sherman, despite being built on the same chassis. Wonder where the weight went? It was armor they
did not put on the M-18. How many tanks have an open top turret? The M-18 Hellcat Tank Destroyer does.
The vehicle is called the M-18
tank destroyer, because the military says it is
not a tank.