I think it's difficult to point at any one, two or three things about Hypercard that caused it to slowly die. It very much died a death of a thousand cuts.

a) It was written quickly by one programmer (Bill Atkinson) who left Apple shortly after and (if rumors are believed) no one else really understood his code.

b) It never really adapted to colour displays

c) It never adapted to variable size displays

d) It had no networking so it couldn't compete with the web

e) It didn't run on Windows or Unix so it couldn't compete with the web

f) It was closed source and proprietary so it couldn't compete with the web

g) The free version of Hypercard was made read-only and lost the ability to write and edit stacks (taking away the entire point of Hypercard)

h) XCMDs were a pain to write but required for almost anything (Hypercard's API were never really extended to include new features as the Mac added them)

i) Apple themselves stopped using it for all help and tutorials on the Mac so people stopped experiencing its metaphors when they first used their Macs

j) Apple moved it to Claris (who killed everything they touched)