Well, the main reason no one uses Hypercard itself is because emulating classic MacOS is a pain and a half.
I haven't explored all the copies, but the ones I have seen tend to be less featureful and/or less user-friendly than the original.
Furthermore, probably the best modern software that's similar to Hypercard in its overall flexibility and power is...the web. Back in the day, Hypercard was it; there wasn't really anything else in that space. Nowadays, there's still nothing that's quite as awesome or matches all its features, but there are things that are much closer and do a lot of the same things that are much more accessible than emulating or reimplementing Hypercard.
The existence of the Web is indeed a big reason. While everybody seems to compare Hypercard to things like Visual Basic, and yes, you could use it that way to make applications, most people used it to create stacks on a topic (a particular band, or Shakespeare's plays, or Japanese swords) that were basically Web sites before the Web.
Emulating Mac OS classic isn't particularly difficult these days. I use sheepshaver with great success. I actually DO use it to run my old hypercard games I wrote in elementary school.
Last time I tried using Basilisk II, I managed to get it to be stable (running System 7 in an emulated...IIsi, I think?), but if I ever tried to change anything beyond adding and removing disks, it just wouldn't run.
I haven't tried SheepShaver much, I admit, because most of the stuff I want to run was written for 68k and runs poorly or not at all on PPC.
I haven't explored all the copies, but the ones I have seen tend to be less featureful and/or less user-friendly than the original.
Furthermore, probably the best modern software that's similar to Hypercard in its overall flexibility and power is...the web. Back in the day, Hypercard was it; there wasn't really anything else in that space. Nowadays, there's still nothing that's quite as awesome or matches all its features, but there are things that are much closer and do a lot of the same things that are much more accessible than emulating or reimplementing Hypercard.