Nadal was still unfit when he played Del Potro in the semi finals, he had tendonitis in his knee and he could barely move around court so of course someone with the same pace on the racquet as Del Potro was going to triumph.
And even if Del Potro might have gone on to challenge for big honours after his 2009 US Open win he was out for ages with that injury to his hand and has come no where near a grand slam challenge ever since. In fact i think this is his first semi final in four years.