Arthas had less reasons to go evil. He was born with a silver spoon in his mouth, an heir to an entire kingdom with everything gifted to him. Anakin grew up poor, enslaved, then had to learn how to use space magic with a weightless space sword and becoming a general at age 19. If anything Anakins fall is more believable considering the weight of entire planets and systems depended on him. Arthas could not even stop 1 kingdom from falling.