Digg was Digg, then V4 launched, and it was no longer Digg. It was really that simple, oh hey I should check Digg maybe there is something interesting went to oh hey I need to find something else to check because Digg is stale, the new UI had much lower information density, coupled with less content meant that there was no reason to go back if you could find a faster feed.

New stuff like reddit, insta, twitter, simply took over filling the urge to hit f5 for latest content.