He said it was due to missing out on other opportunities (now perceived to be more valuable) as a result of entering and dedicating his time to the domain industry.
But hindsight is 20/20, so it is a lot easier to feel that way after various events have occurred, regardless of whether or not there was an objective basis for predicting such outcomes at the offset.
However, as per
@pqr's post above it looks like he has re-opened his account, so I assume he no longer thinks starting domaining was the worst decision of his life.