The 49ers weren’t going to play Detroit on Thanksgiving Day because they are a Pacific Time Zone team. (The Lions haven’t hosted a team located west of the Central Time Zone on Turkey Day in over 30 years.)
The 49ers probably weren’t going to play Dallas on Thanksgiving Day during the Montana/Young era because 1) they had to be scheduled to play at Dallas in a given season, 2) CBS and NBC (and later Fox and NBC and then Fox and CBS) would alternate televising Detroit and Dallas, meaning the 49ers would only be a possibility to play on Turkey Day in Dallas every other year, and 3) the NFL gets huge TV ratings on Thanksgiving for the afternoon games because of the captive audience, and they weren’t going to waste a marquee 49ers/Cowboys matchup that would get huge ratings whenever it was played by playing it on Thanksgiving. The only time the 49ers might have played at Dallas during the Montana/Young era was when the Cowboys were down (1987 to 1991, and really only 1989 or 1990).
I suspect Jets fans can relate.