I think this project failed for a number of reasons:
Lack of clear scope
Here's what I mean: "Q&A site for 3D graphics creators"
What is a "3D graphics creator"? For me, as a programmer who deals with game development, a "3D graphics creator" is a modeller and/or animator. This might also extend to texture artists and possibly shader artists.
Yes, absolutely questions discussing techniques covered by Siggraph would be accepted and welcome!
That doesn't sound like what the site's scope is to me. That is, it is not clear to me that discussions of SIGGRAPH papers, particularly those that focus on programming techniques, would be welcome on a site for "3D graphics creators".
Similarly, application and toolchain programmers are not "3D graphics creators" from my perspective.
My point is simply that the site's scope was not communicated well. Many people who might have wanted to participate were thus warded off. This led directly into the next problem:
In order to make a Stack Exchange site, you need people who want it. When even the person who proposed the site originally got tired of it before it hit beta, there are issues.
Most successful SE sites were initially built around a core of people who wanted it to succeed. These people had the energy and drive to go out and try to make it a reality. They were also connected to various communities that already existed, so that they could draw those persons to the site.
You cannot rely on Google drawing people to the site; not when it's starting out. You need to have "boots on the ground" to make a site a reality.
And that's the purpose of the private beta: to prove that people who are already involved with the site are sufficiently energized. To prove that they are connected enough for them to hand out invitations to people who would become the first wave of converts.
That didn't happen.
Ignoring the question of whether there truly are millions of people who would fall within this scope, the size of the audience alone simply isn't enough. You can have a product that anyone might find useful. But without actual marketing, it's just not going to work. Nobody who might want it will know about it.
The private beta is a test of the community's ability to market, to build the userbase through direct contact with potential users. Without that basic level of marketing, the site would surely die out.
You cannot simply build something and expect people to show up, no matter how wide the audience is.
But there's one other problem:
The alternatives work
Stack Overflow came about for two reasons:
Forums have a major problem with help vampires and other creatures of the night who eventually cause experts to retreat.
Experts Exchange, an attempt to deal with the aforementioned problems, was taking some unpopular steps.
Help vampires exist on graphics forums, certainly. But they're not nearly as pervasive and persistent as they are in pure programming circles. It seems to me that such persons become more quickly dissuaded from pursuing modelling or other things as a hobby when they see how much work it is than for hobbyist programmers.
Stack Overflow fulfilled a real need among the programming community. A need that people really wanted filled. This 3D graphics site would fulfill a need, but it isn't so drastic of a need. It just isn't as badly needed.
Without the driving force of that need, you now need some way to convince people to abandon their forums (by spending more leisure time on this site) for this Q&A stuff. That's not an easy sell.