I would suspect that it’s because instagram doesn’t allow direct links without an account. For instance, when I go to that link (because I don’t have an account) I get redirected to this page:
When I go to that url in iOS safari, it tells me that the url is not playable. That would explain this. It simply could be an encoding that it can’t handle.
Another possibility is that the location it is being served from is not sending the correct information to allow streaming. I see that if I download that url, I get the big bunny movie and it does play.
Thanks Greg for confirming, that URL worked. I tried several url’s none of which worked even though they showed in the HTML viewer so I am not sure on the difference maybe back to your other suggestion of an encoding problem.