1. You need really good surface contact to get the diffraction on, so you can only really apply it to flat, uncurved surfaces
2. You need to apply it while the chocolate is hardening. If you're using a mold, you cast the chocolate upside-down so only the base is exposed during setting. You can't diffract the part people will actually see.
3. The effect is really fragile. Chocolate melts at around body temperature, so if you hold the piece for too long, the diffraction disappears. I've been able to transport diffractive chocolate in a padded box but don't think I could wrap the chocolate directly.
I can get a pretty decent temper on chocolate and I make bonbons once in a while, but I have still found that you can lose fine surface details with a temperature that is a comfortable "room temp" for humans.
maybe a thin sugar shell around regular chocolate
[1] https://www.npr.org/sections/thesalt/2014/06/14/321816570/ho...
In all seriousness though, this would be pretty cool and I'd love to try it. I suspect that some of the problem is just that companies tend to constantly push to deliver the bare minimum while charging the most. Holographic chocolate would take more money to make and companies are always trying to charge you as much as you're willing to spend for chocolate while giving you the lowest quality/effort product you'll still pay for. While I wouldn't expect to see something like this showing up in the candy aisles of your typical grocery store smaller fancier places will be willing to do it and charge a premium
https://www.brunnershop.com/en/Frame-Moulds/Holographic-choc...