Jobs had the balls to do to Hypercard what Adobe didn't do to Flash. If Hypercard was so amazing then why did products like Supercard fail. It matched Hypercard's feature set and was able to run on Windows and Macs.
HyperCard would have been a great platform if it was aggressively iterated. Despite being way ahead of its time in 1987, it died because it stood still and the world eventually passed it by.
Aggressively iterating it would make it too complicated to use for beginners which would also cause it to lose popularity.
I mean, there's a lot of potential reasons, maybe because it didn't have the marketing budget of a multi-[million|billion|trillion] dollar company behind it? Maybe it tried to be too much? It's tough to say.