It's kinda the opposite actually. iframes didn't provide sufficient security to do the sort of things Google wanted to be able to do with them, so they had to design a new standard with better protections: https://github.com/WICG/portals/blob/master/explainer.md#why...

I don't understand the cynicism people have for this idea. All they did was say "wouldn't it be cool if you could have nice animations in between pages" and built a proof-of-concept. It's not a finished product. They aren't forcing it into a standard. It's a demo of something that would be cool.

So why the heck are people opposed to that?