Yeah its just tradition for Cowboys and lions to have games on Thanksgiving day, not sure the exact origin bc its happened forever. Don't think they ever did third games until regular Thursday night football became a thing on normal weeks
The Detroit Lions negotiated a deal with the NFL and NBC Radio in 1934 after moving from Portsmouth to Detroit as a way to get stands into the stadium, thus birthing their involvement in the Thanksgiving tradition.
The Cowboys were a similar story, since they were an expansion team, Tex Schram saw an opportunity to get his team in the national spotlight, and since they were marketed as "America's Team", it made sense to put them in Thanksgiving games. This began in 1966.
So basically, for both teams it was a marketing maneuver in their early going that's just... sorta stuck.
