Probably because:
1) HN wasn't that big (or existing) at that time.
2) Now you can also see the code, it's not in some opaque swt archive.
3) You don't need a proprietary Adobe IDE to code it (yes, you could do without for Flash too with some bizarro third party tools. Talking about the mainstream).
4) Everybody hates flash anyway.
I suppose this is true, though the same could have been said for flash before Javascript's recent strides, and I don't recall anyone being particularly excited about similar demos implemented in actionscript.