Because there is no other plausible explanation. Rejecting an ambassador is a pretty serious act in the diplomatic world. It is not done because you don’t like his haircut or the color of his tie. Beyond which there have been plenty of leaks from “highly placed Vatican officials” that leave no doubt whatever as to the reason.
What “highly placed Vatican officials”?
Sorry, if that same answer was given to certain other Catholics on here about something the pope said that was not seen as a positive thing for him, they would question the source.
In my mind, there could be another explanation. I’m not up on Vatican rejections of ambassadors, but perhaps there were other reasons in the past that could apply here.
And another thing. If this rejection was because the man is openly gay, why not state it? Let’s stop pussy footing around and speak boldly. That is if the Catholic Faith is what is most important.