See top illustration.
The top rear of the tank where the engine is.
Thats where it was hit.
No RPG could do that unless it was hit exactly at the top rear.
If RPG was fired on level with the tank there is very little chance of it gettin taken out, nearly impossible.
Top rear is very weak in armor thus why the turret is extended in the M1 design.
M1 is an awesome tank but when it does not have proper support in a bad position on a field its a sitting duck.