Conspiracy theories and RNG for sure - it averages out somewhere in the middle. Most of my friends who play Diablo play it a fair amount, ie. 10-20 hours/week and they tend to get a good amount of drops. One of my friends only plays 1-2 hours per week if that and he's never gotten any good drops, another plays the same and got 3 legendaries in one run the other day... lucky him .. but that's all it is, luck
The only thing I know for sure is that playing through on normal generally yields some decent items for your level but from Nightmare on up its crap crap crap.