The reply to Sholom Aleichem is Aleichem Sholom.
One reply to Yeyasher Kochacho is Kochacho Yeyasher.
Why then have I not heard the reply to Mazel Tov to be Tov Mazel?
I heard an explanation.
Aleichem Sholom is not grammatical.
But we overlook the lack of grammar in order to start with Sholom and end with Sholom.
We could say the same for Yeyasher Kochacho and Kochacho Yeyasher. We want to start with Yashrus (being straight and honest) and end with Yashrus.
But we do not want to start with “mazel” in its meaning as “astrological sign” and end with it.