The reason is turtle position is a weaker position compare to the other positions.
While it is convenient to turtle in order to give up a guard pass points which is 3 points, your opponent still scores an advantage.
Also, any "sweeps" from turtle is NOT a sweep. But it is a reversal as turtle is not considered a guard. You do NOT get 2 points for turle.
This week we covered, fundamentals of turtle, reversal from turtle, clock choke, banana split and back take from Turtle.