I think part of the reason it didn't succeed has to do with the depth versus the breadth of the field. Two of the most successful sites are mathoverflow and stackoverflow. Math and programming are both broad fields. If someone in the field has a question about mathematics or programming, it is reasonably likely that somebody else in the field can answer it with relatively little effort, as long as the question isn't in the first person's area of specialization. Theoretical physics (at least the part of it that had representatives participating on the site) is very deep, but not particularly broad, and so for a typical question, it might be that nobody knows the answer, or it might take a long time to figure out the answer. You certainly get questions like this in math and programming as well, but they're quite a bit less common.

I think part of the reason it didn't succeed has to do with the depth versus the breadth of the field. Two of the most successful sites are MathOverflow and Stack Overflow. Math and programming are both broad fields. 

If someone in the field has a question about mathematics or programming, it is reasonably likely that somebody else in the field can answer it with relatively little effort, as long as the question isn't in the first person's area of specialization. 

Theoretical physics (at least the part of it that had representatives participating on the site) is very deep, but not particularly broad, and so for a typical question, it might be that nobody knows the answer, or it might take a long time to figure out the answer. You certainly get questions like this in math and programming as well, but they're quite a bit less common.