I have answered your question, though not spent time on it, as it is only tangentially related to my point.
I’ll elaborate.
The short answer is I don’t know. The longer answer is we don’t know it could be because of mikva it could be because of yichos it could be because of Simcha, it could be something else (the arch hashulchanha, gra and others don’t seem to like any of those). While definitely an interesting question it just can’t be answered with certainty. But more to the point, as long as it can be justified and/or has rabbinic backing,it doesn’t really matter lemaaseh.
Now your turn. Because whether the rema would have come to his severa without the minhag, I’d PRECISLY my point.