Whatever the reasons the Aztecs (and others) lost, lack of trying and motivation certainly wasn't one. It just plain tends to be kind of difficult to fight a technologically and methodically markedly superior invader who can enlist major support from your local foes, all the while suffering from very serious new diseases and internal troubles...
Did you know, when the estates and suchlike were divided after the conquests, there tend to be curiously many specimen of the previous ruling class in the surviving rolls ? Opportunism and abandoning sinking ships, it seems, are something of an universal human trait.