Not saying that Digg aren't clowns but the traffic balance makes a huge difference. It's not implausible that Digg has an enormously bigger fraction of writes to reads than SO does which changes the scaling dynamics considerably.

Edit: To expand I'd imagine SO serves a lot of static "how do I" google hits for logged-out users which is about as easy to serve as it gets. Digg puts a higher emphasis on the logged in experience and commenting which reduces the ability to cache.