The Devil loses because he is made to lose.
It is a sad little fact of the human nature that we need someone 'who we can always beat up.' We need a bad guy and because of some sort of literary rule handed down from the start of time: The good guys always win. The story is written for us to feel with the good guy, for us to put ourselves in their shoes and for us to relate to thier situations. If this happens and the good guy looses, it makes us feel bad about ourselves on some level. Go figure, most humans are this easy to influence! Personally I find it sad the human mind can be influenced in such a way but we want the people we relate to be victorious simply cause it makes us feel better about ourselves.
Because the devil is cast as the bad guy then he has been made to lose before the whole story even begins. Doesn't matter if he was the best damn fiddle player out there, that managed complex riffs that would normally break your wrists, he would still have lost simply because he was viewed as the bad guy. It wouldn't matter if Johnny, in this example, couldn't play a single string correctly the non-existent judges would of given him the prize. Simply cause the Devil was the Devil and we all know the bad guy never wins....
I've often found it far more entertaining if there is the possibility of the people we relate to loosing in the end. Not only is it far more realistic, most times the good guys lose in real life, but it is more suspenseful and doesn't end up insult our intelligence like most movie do. Face it, when the good guy is outnumbered 1000 to 1 he should become nothing more then a smear on the ground. None of this bullshit slow motion filming where he manages to take down ten people with a single round, dodging a hail of bullets and coming away with out a single scratch....
I keep getting reminded of watchman... man vs being with god like powers. Won't say how it ended for those who don't know. Yet any contest with the Devil in it, another being with more then human power, should end the same damn way.