I'd guess it's mostly the pile of work completed...
Based on the 6.0 version number it has been an ongoing project for awhile. Decisions such as discontinuing a project as large as a distribution at version 6.0 tend to take time and not be made lightly. It's self-evident, but if a project is to end, and at least for individual finite human beings they all do eventually, there must be a time at which that end comes. Just like death itself, unfortunately that end doesn't always come at a convenient time, even everybody knows it's coming "someday".
So I'd guess he's just no longer in a position to keep it up, but awareness of that came slowly, and once a decision was made, he both already had some work put into the next version, and felt he had an obligation of sorts to his users, to at least put out this one more.
So here we are. But with luck, as you mentioned, someone else could pick it up. Or, perhaps as with LWN's "this is the end" article several years ago, the release and declaration itself will bring enough new interest that instead of it being the end, it's a new beginning.
Time will tell. If it's of enough interest to enough people, one way or another, it'll continue, either directly or as a fork. Not only is that an oft stated benefit of FLOSS, but in the FLOSS community, it's as certain as death itself. The only reason a FLOSS project ceases to exist is because there's simply /not/ that level of interest, at least not to enough people.
Duncan (who had a neighbor die the other day, after I had just talked to him and happened to get the power saw back that he had borrowed, the day before, so death and ends, and the nature of human experience in respect to them, is something I've been thinking a bit about the last few days...)