Order of priority in hybridization of d-orbitals
The rotational frame of reference which defined which axis is the z-axis is changed such that $d_{z^2}$ and $d_{x^2-y^2}$ are hybridized first is a convention / practice.

In theory (meaning mathematically), you should be able to get equivalent geometry if you used the other d orbitals for hybridzation.

Basically the donut thing is weird enough we try to fold it in so we can deal with the more normal cloverleafs as leftovers.