Congress has not only the right but the fiduciary duty to regulate interstate commerce. It's very much written into the constitution. The problem is that legal weed is so huge now that it can't prohibit it, or even regulate it, since it has failed to do so now for decades as individual states legalized both recreational and medical marijuana.

Congress is just so broken at that point that states for decades now have ignored Federal regulations and found ways to work around it. Congress has made itself irrelevant to some extent. The country is moving forward without it.