HyperCard was the right tool at the right time, but times change. HyperCard was super powerful with a medium learning curve, but at a time when personal computers were expensive and still an enthusiast (or school) device, users were willing to go further to do more. As adoption increases and prices decrease, there were fewer who wished to produce with the machine and more who wished to consume. HyperCard's market became unprofitable, and it went away.
People mention Flash, and that's a fair replacement, but I would argue it had a similarly steep curve, and the driver there was money - making Flash animations would make you money. Then Apple decided we were done with Flash, and here we are.
The difference between HyperCard and Flash was that HyperCard was for "computer users" who could take the demo card file stacks or whatever and customize them. Flash was for artists and animators who realized "wait I can link this to this" and suddenly they were game/multimedia developers.
Now everything is just for professionals who have already decided what they want to do.
I think PowerPoint or Keynote are better analogs for HyperCard. They all use a "deck of cards" metaphor at their core. HyperCard came on the scene when making GUI applications was still "new". There was so much low-hanging fruit. HyperCard doesn't exactly exist today because the "winners" in each category now exist as full-featured apps, like PowerPoint or Keynote for presentations.
People mention Flash, and that's a fair replacement, but I would argue it had a similarly steep curve, and the driver there was money - making Flash animations would make you money. Then Apple decided we were done with Flash, and here we are.