Why does the Torah call the mitzvah of the Red Heifer a statute?

(19,2) This is a statute of the Torah which Hashem commanded, saying: Speak to the children of Yisrael and they shall take for you a perfectly unblemished cow upon which no yoke was laid.

Behold, we find many mitzvos in the Torah whose reasoning is not explained and thus are also statutes, yet the Torah does not refer to them as such. So why is the mitzvah of the Red Heifer different?

The answer is that Chazal taught that this mitzvah was to atone for the sin of the golden calf - "let the mother come and clean up the mess made by the child". Yet Chazal taught on the posuk in Shemos (15,25) "there He made for them a statute and a judgement" that the mitzvah of the Red Heifer was commanded whilst they were in Marah. And since when they were in Marah they had not yet sinned with the golden calf, it was impossible to give this reason for the mitzvah. Therefore, the Torah called it a statute.