Once that window closed, the license expired. Harmonix could no longer legally offer the export.
When Harmonix secured the rights to a song for a video game, they did not own the song. They licensed it. Crucially, these licenses had time limits. Usually, the contracts allowed the songs to be sold as DLC or exported for a specific window of time—often around five years. rock band song export key
Scroll down until you find a specific tile labeled "Exports" or "Previous Game Exports." Once that window closed, the license expired
The only exception is the digital sale of individual tracks. Currently, Harmonix has managed to relicense and re-release many of the Once that window closed