Youtube embed video not working in Safari

I had an issue today about embeded a youtube video. I use the code they give when you are on a video page for exemple:

<iframe width="560" height="315" src="http://ift.tt/1aiwcZ8" frameborder="0" allowfullscreen></iframe>

But in safari if I inspect element, it shows that there is nothing loaded in the iframe.

So I've just changed the secure transfert protocol by the normal transfert protocol.

<iframe width="560" height="315" src="http://ift.tt/1ckbyp5" frameborder="0" allowfullscreen></iframe>

Now it works, so do you know why safari block the secure transfert protocol for youtube embed videos ?