I don't why but I seem to recall that the show wasn't originally envisioned as the huge and legendary event that it became. Over time, as casting began to fall into place the already excited word-of-mouth became more and more rabid. And then I think an album deal was made. And then the PBS deal was made. In the documentary Carol Burnett says something like they all thought it was going to be an easy gig but then they realized/learned that everything was going to be documented and recorded.
I could be wrong. But there is no question that this is one of the most legendary events in Broadway history.