Because we say that marriage is marriage, a thousand miles of marriage is holding hands, and the red line is holding two people's wrists, so it is not on the ankle.
We can also see in many film and television works that if there is the role of Yue Lao, then the tangent line is also a red line on the wrist of the character.
And it will be more beautiful if you tie a red line on your wrist to match your marriage. Obviously, ankles don't have this sense of beautification. Moreover, it doesn't make much sense for us to say that something is tied to our feet, which is usually locked, such as fetters.