Probably mostly the fact that this was started (according to the website) in 1996 and essentially frozen in 2004, which I believe predates the public announcement of Asymptote.

Does Asymptote get much use these days? I thought it was pretty cool, though a bit hard to wrap my mind around, when I first tried it, but nowadays it seems that attention has moved on to TikZ.

Low-level, cool stack manipulation, and it predated Asymptote (2003) by being released in 1984.