Instagram pictures don't show up on Twitter because Facebook blocked it. They did that in response to Twitter blocking Instagram from finding contacts via Twitter. Its FAANG in-fighting from like 8 years ago that I'm still kind of surprised hasn't been sorted out by now.
I listened to a podcast that included this story, it was a Facebook decision. Mark phoned and gave them a few hours heads up that 'Instagram' had decided not to allow the embeds any more. They actually got it delayed 24 hours or something like that to avoid breaking Twitter.