Because the Mint has the legal authority to mint such a coin. But the Treasury department cannot issue debt in excess of the debt ceiling. Congress can fix all

Because the Mint has the legal authority to mint such a coin. But the Treasury department cannot issue debt in excess of the debt ceiling. Congress can fix all this with a stroke of a pen, but eliminating the debt ceiling or lowering appropriated spending below what the debt ceiling would require. This is just a cheap trick to allow the executive to bypass the debt ceiling/government shutdown shenanigans.