I think it might have to do with Congess' power to regulate what sorts of things may enter the channels of commerce, what things can be allowed to enter commerce (whether on highways, by boat, train, etc). It's been recognized for a long time (since about the late 19th century) that Congress has the power to prhibit certain, usually noxious or harmful items (like diseased cattle, bad food, contaminated waste rags, etc.) from enterring interstate commerce, in order to keep the channels of commerce squeaky clean and pure for commercial traffic in legitimate items. Congress may also, according to Lopez, keep commerce free from "immoral and injurious" uses, which means they can regulate those who profit from commerce or prohibit certain items from entering commerce when those items are the result of a practice of which Congress disapproves. The examples cited in Lopez are Heart of Atlanta Motel (Congress can prohibit a motel which profits from interstate travellers from discriminating on the basis of race) and Darby (Congress can ban items from interstate commerce which are made under labor conditions they deem to unacceptable).