Link: https://pe.usps.com/text/csr/PS-334.htm
A 2014 ruling states:
"While video games may be read on computers, unlike books or films eligible for Media Mail prices, which predominantly further educational or informational goals, games, including board games and games in an electronic format, are used primarily for entertainment and they do not serve the same purpose as books, films, or other qualifying Media Mail... In conclusion, video games, whether on CD-ROM, diskettes, or similar software, regardless of form, or playable systems including computers, do not meet the standards for Media Mail."
Unless there was a more recent decision that overrides this, USPS pretty much spells it out here.