You cannot display a lot of websites inside an iFrame. Reason being that they send an "X-Frame-Options: SAMEORIGIN" response header. This option prevents the browser from displaying iFrames that are not hosted on the same domain as the parent page. This is a security feature to prevent click-jacking. Some details at How to show google.com in an iframThate?
That last sentence points to: https://stackoverflow.com/questions/8700636/how-to-show-google-com-in-an-iframe
It seems like the change you are looking for occured on the source of what you want to iframe ...