I'll take a stab at this. Article I of the Constitution enumerates the powers of Congress. In particular, Article I, Section 8, Clause 3 grants Congress the power to "regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes". (Notice the power to regulate intra
-state commerce is omitted.) However, FDR's New Deal legislation really pushed the limits of this power, and in Wickard v. Filburn
(1942), the Supreme Court acquiesced and let stand a law that limited how much wheat a farmer could grow even if the wheat never entered interstate commerce and instead was consumed by animals raised on his farm. In Gonzales v. Raich
(2005), the Supreme Court held that Congress can criminalize homegrown cannabis for personal consumption even if state law allows its use for medicinal purposes. (Recall that in the 1920s, a Constitutional amendment was necessary for a national prohibition of alcoholic beverages.) Today, there are lots of laws whose only jurisdictional hook is that they regulate activities or products "in or affecting interstate commerce", and the "affecting" part is taken extremely liberally.