Perhaps because it wasn't just a rock - first and foremost it served the function of a well.
The Gemara in Taanis 9b clearly states that it was brought in Miriam's merit and Rashi writes over there:
...בארה של מרים - סלע וזבין ממנו מים והיה מתגלגל והולך עם ישראל
The Well of Miriam - a rock and water flowed from it and it would roll and (lit.) go (i.e. ...