Probably because our U.S. Constitution/Bill of Rights is the Supreme law of the land. That means nothing can be deleted from it...though amendments can be added if they are in harmony with the whole document and no conflict is found.
Congress can pass no laws, even laws Congress wants to pass which are based on treaties and the enforcements thereof which are not in pursuance to the Constitution. (Article VI, para 2.)
Article I, Section 8 declares that Congress has the power to grant Letters of Marque and Reprisal, [but it has no power to nullify that power, either by legislation, treaty, or amendment]. IMO. Looks to me like this article is still operational.
If it's in the Constitution, why do we need this bill? Isn't it redundant? And weren't the Contras some of our Marque and Reprisal Guys?