Why the Tiara and Not Mitre on the Papal Arms

"Indeed, the ministry of the primacy involves, in essence, a truly episcopal power, which is not only supreme, full and universal, but also immediate, over all, whether pastors or other faithful."

-Some Aspects of the Church Understood as Communion, c. 1992, by the Congregation for the Doctrine of the Faith.

[cf. Dogmatic Constitution Pastor Aeternus, ch. 3; Dogmatic Constitution Lumen Gentium, 22, par. 2].