I think Apple killed HyperCard because of the performance issues, and because it didn't fit into Jobs' vision. Once they settled on NeXT for OSX, the NeXTStep environment had a lot of cutting edge rapid development features but with the speed of compiled code. They didn't want to develop two separate environments, so HyperCard got the axe.
It is too bad. I would love to see HyperCard alive again.
I also really hate waking up before the sun comes up.
Fixed that for ya Chelsea.