When the Cowboys moved out of the Cotton Bowl in 1971, they decided, along with the improvements of color TV, that they would wear white at home to give the viewing audience a variety of different uniform matchups for every home game. Instead of blue vs white, it was white vs whatever the opposition’s team color was.
They’ve kept that tradition to this very day. The only time the Cowboys have worn blue at home was when they decided to wear throwbacks, which they have done in past Thanksgiving Day games.
However due to the new helmet rule that prevents them from wearing their traditional throwbacks and the Cowboys’ already informing the league back in the summer they’d wear their dark color on Thanksgiving, they have decided to wear their blue road uniforms. They could have worn their throwbacks with their current helmets, but that wouldn’t look too good.