After Gabriel came to Zechariah and said Elizabeth will conceive a child, Zecahriah said “How shall I know this? For I am an old man and my wife is advanced in years”.
After Gabriel came to Mary and said she will conceive a child, Mary said “How can this be, since I have had no relations with a man?”
Mary and Zechariah both doubted Gabriel’s message, yet Gabriel chose to punish Zechariah by making him speechless and unable to talk. Mary had no such punishment.