The A400M is a turbo prop, ie a propellor aircraft which uses a jet engine to turn the prop. The prop on the outside is not an external version of the spinning compressor blades on the inside of the jet engine. They are still there inside the casing and under the cowlings. The blade shape is the result of advances in aerodynamic science and the manufacturing advances that have happened over the years.
Why not jet engines? Someone who knows more than me will be along soon to explain, I'm sure!
CG