Why crabs can't walk straight?
Crabs belong to the crustacean family. And they consists 10 legs. So, they have been called as decapods. Like the human, crabs cannot walk forward because the human has knee and it can bend forward. So, it makes them walking straight easily. But crabs legs are attached to the side of the body. That is why the cannot move forward.
Not all crabs move sideways, but there are few species which can walk forward due to their long body shape.