In some cases it has to do with the operating system cooperating or not. Modern operating systems don't allow certain things to work due to restrictions in what apps are capable of. Consider a Smalltalk OS, where you can dynamically link objects across "apps" in interesting ways (see Alan Kay's "Tribute to Ted Nelson" where he shows a demo of this). You simply cannot do such a thing with mac/windows/linux.

In the case of Hypercard, I cannot say whether or not this is the case. It could be that Hypercard is absolutely possible today. But I wouldn't doubt it if it was somewhat unusable on modern systems due to this "cooperation" issue I mentioned. It may need buy-in from other apps and/or the host OS for it to work fully as intended.

For another example, consider emacs. Emacs effectively gives you the lisp-machine experience, but the problem is it isn't integrated with the rest of the system. You sort of have to live in an emacs bubble. With hypercard, you could surely get it running, but would you be in a bubble? Ideally you could use hypercard to script the rest of your system as well.

What we should want is something like a "card" that could "link" to a specific cell in a spreadsheet, as an example. Or a card that could open a PDF to a specific page. The more the rest of the system "plays along", the more powerful something like hypercard could be.