To provide power in engine, piston needs to be moved; from top to bottom and bottom to top.
In two stroke engine, piston has to move 2 strokes to produce power stroke once in engine. This requires one rotation of crankshaft. In other words in 2 stroke engine the crankshaft revolves one time to complete its working cycle.
In 4 stroke engines, the crankshaft does 2 revolutions to complete its working cycle. In other words, to obtain power once in 4 stroke engine, 4 strokes of piston are required. That means 2 rotations of crankshaft.