Pardon me if someone else covered why Cowboys didn’t play Thanksgiving in 1975 &1977:
Since 1966, the Cowboys have been featured on Thanksgiving Day in every season except for 1975 and 1977, when Dallas was replaced by the St. Louis Cardinals.
The Cardinals had moved from Chicago to St. Louis in 1960, and they didn’t make the postseason once until 1974. After a couple of losses for the Cowboys on Thanksgiving in 1972 and 1973 and backup quarterback Clint Longley took over for an injured Roger Staubach in 1974 and led Dallas to a come-from-behind, 24-23 victory over Washington, the NFL decided to give America a break from the Cowboys on Thanksgiving Day.
The Cowboys were back in the mix the following season, and the Cardinals hosted Dallas in St. Louis. The Cowboys got the best of the Cardinals, winning 19-14.
In 1977, the Cardinals hosted Miami and this game may have been enough reason for the NFL to scrap the whole Cardinals on Thanksgiving Day experiment. Miami beat St. Louis with a ridiculous final score of 55-10, and that was the end of the Cardinals replacing the Cowboys as a featured team on Thanksgiving.
Nobody seems to know why the NFL decided to make a switch and feature the Cardinals instead of the Cowboys for those two games, but Dallas is certainly entrenched as a Thanksgiving staple today.
https://www.sbnation.com/2016/11/23...-cardinals-on-thanksgiving-twice-in-the-1970s
The Cowboys hosted a Thanksgiving game annually through 1974. But Dallas didn't cement itself as host until after the NFL's failed attempt to market the St. Louis Cardinals and head coach Don Coryell’s high-flying offense. On Thanksgiving Day 1975 and 1977, the Cardinals hosted games. On Thanksgiving 1976, they played at Dallas. St. Louis lost all three games.
After the 1977 game, NFL commissioner Pete Rozelle asked Schramm if he wanted to take back the game in 1978. "I said only if we got it permanently," Schramm told the
Chicago Tribunein 1998. "It’s something you have to build as tradition. He said, ‘It’s yours forever.’”
https://www.history.com/news/thanksgiving-football-nfl-lions-bears-tradition#
Note: 1976 Thanksgiving game with StL was actually played in Dallas.