Why Did the Detroit Lions Start Playing on Thanksgiving?

They needed a crowd:

In 1934, radio executive G.A. Richards bought the Portsmouth, Ohio Spartans NFL team, moved them to Detroit, and renamed them the Lions.  Unfortunately for him, nobody in Detroit cared much for watching the Lions.  Despite winning all their games but one before Thanksgiving, having several stars of the day, and one super star in Earl “Dutch” Clark, the average turn out for each game was only around 12,000 people.
At the time, it was fairly traditional for various football programs in high schools and colleges to hold particularly significant games on Thanksgiving.  So Richards decided to try to bring this same tradition to the NFL, convincing the NFL to allow the Lions and the defending World Champion Chicago Bears to play for the Western Division championship on Thanksgiving.
Richards then used his considerable influence in radio to convince NBC that they should broadcast this game on the radio all across the United States, something that had never been done before for an NFL game.  The game ended up being a huge success, being played at the University of Detroit Stadium in front of a sold out crowd of 26,000 fans and broadcast across the nation on over 94 different radio stations.  In the end, the Bears won 19-16, but the game was such a success, as far as ratings and fan turn-out went, that Richards fought to be allowed to continue having the Lions play on Thanksgiving going forward and to continue to have that games broadcast out on the radio nationwide.

An additional bit of Thanksgiving football information:

 Football was actually traditionally played on Thanksgiving long before the NFL ever existed.  As early as 1902 it was common for such leagues as the National Football League (not to be confused with our current National Football League) to use Thanksgiving as their championship game day.  Interestingly, this league was funded by Major League Baseball.  Another league, called the Ohio League, liked to match their best teams together for Thanksgiving games.  Many other such leagues also did the same thing.  As noted, it was also common among many colleges and high schools to hold annual Thanksgiving football games.

More on the Portsmouth Spartans here.  More Thanksgiving facts here (h/t Ritholtz).  Every year, the Lions game rouses grandpa to say that back in the day, the Lions played the Packers every year.  From 1951-1963, he's right.  But in other years, not so much.  Since they are playing this year, he might or might not say it.  I'm counting on him asking what kind of cheese we're eating (Havarti), then telling us he's never had it before, even though he has, each of the last ten years or so.  Finally, I predict he'll be asleep within 25 minutes of finishing supper.  God bless grandpa and the predictability of our holidays.

Update:  I went down to mom and dad's at lunchtime, and grandpa was already there.  We had something to eat, then at 12:47, grandpa said that every year Detroit played Chicago (oops, I had the wrong longtime rival).  That was before we even turned on the game.  At 1:15, grandpa was taking a nap.  When I left to feed the cows, mom hadn't put the cheese out yet.  I'll have to head back down and partake of that holiday tradition.