I’ll ask the same question here that I asked in a similar thread: How do we know that the reason why he was rejected was because he was gay? So far there have been no official statements: the rejection is a rejection because there has been no acceptance.
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.