It's probably a combination of things, including the metals used and, as you say, the oils and the engine design. Also, you would probably have to include computers too. With computerised sensors, the driver can be alerted to problems without having to open the bonnet.
Of course, you have asked the question "why do they last longer" but I don't actually know whether they do or not. Is this an observation of yours? If so, is it accurate? Do you have figures for it? Maybe people are just not keeping their cars as long these days. Also, the "old clunkers" were taken out of circulation fairly recently with some of the government initiatives to kickstart the economy at the start of the recession.