Probably because almost every industry who exports is liable to retaliation for the steel and aluminum tariffs.
Actually the opposite is true...every American exporting industry will likely see a boost as a result of Trump's willingness to take this action.
Countries that want to export to the US on favorable terms will now have to demonstrate comparable purchases of US products.
That's the power that comes from controlling the largest consumer market in the world.
No country will take advantage of the United States anymore.
Well, that and it increasing the cost of steel, which will increase the cost of their products and reduce their sales - regardless of who they buy the steel from.
That said, if tariffs are used tactically, and the diplomacy is good, brief tariffs can help in the long run. In the short term - or however long the tariffs last - its like giving a tax break to one guy and raising the cost to everyone else.