Looking at Alexa's numbers for a rough comparison of pageviews it looks like Digg gets about 10x as many as SO:
http://www.alexa.com/siteinfo/stackoverflow.com#trafficstats
http://www.alexa.com/siteinfo/stackoverflow.com#trafficstats
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.