Digg failed fast due to people already using reddit. Many users had an account on each by the point of the big update. Enough were giving up on Digg for earlier changes.

Digg kept trying to find better ways to monetize, but eventually just gave up on keeping its own identity. By the time Digg released the big UI change, many users just stopped using Digg and used their reddit accounts. Many did have to create new accounts, but reddit was functionally better Digg by that point.

So what made Digg fail fast was due to it already being on the ledge. Digg chose to jump as opposed to get pushed off. Reddit didn’t have a strong alternative coming up like Digg had.

I guess I’m mostly rambling, but Digg was set up to fall already. It just decided to go for it. And reddit was so good for so long that alternatives never built up a users.