Why did alfonso lopez jr court case become an issue dealing with federalism?

The Gun-Free School Zones Act of 1990 (GFSZA) made it unlawful for any individual purposely to have a gun at a place that he knew or had sensible reason to accept was a school zone. Alfonso Lopez, Jr. (D), a twelfth grade student, disguised and stacked handgun into his secondary school and was captured and accused under Texas law with firearm possession on school premises. The following day, the state charges were expelled after government specialists accused Lopez of violating the Act. 
The District Court denied Lopez's movement to expel the arraignment, inferring that the GFSZA was a constitutional exercise of Congress' power in accordance with the Commerce Clause of Article I. The Fifth Circuit switched, holding that the Demonstration surpassed Congress' power under the Commerce Clause and was along these lines illegal. The Supreme Court conceded cert.