The WT equates it with "Harvest Festivals." Here's other reasons from a 76 Awake:
"Today a growing cry is heard against Thanksgiving Day practices. Many feel that current attitudes and customs make it a mockery to call it a “Day of Thanks.” Why so?
In much of the United States, Thanksgiving Day is the beginning of the Christmas season—a ‘holiday period’ extending through New Year’s Day, January 1. So Thanksgiving (now the fourth Thursday in November) becomes the signal for the commercial world to press for what is called the ‘buying binge.’
Further, for many the day is becoming one for ‘saturating’ the public with sports events. The National Observer told of a man who insisted that his wife quickly feed him during the “half-time” break in the football game. “So after the poor woman had worked many hours preparing the turkey and trimmings, the husband said grace, ate his dinner, and was back in front of the television set—in nine minutes.”
While this is an extreme case, the increasing emphasis on sports and commercialistic parades has led more and more people away from any attitude of thankfulness. But how has the “secularizing” of the day, as it is politely called, come about?
Of course, for many the holiday is still a time of family reunion. And there remain those who sincerely view this day as one of thanksgiving to God. But with the rising tide of sports, the frequent gluttony and drunkenness, the trend definitely is in another direction. For a growing majority, having a special meal is the extent of “celebrating” Thanksgiving...
So on November 25, the declared “day of thanks” in 1976, personal decisions need to be made. Dedicated Christians certainly will not want to convey to others the idea that they believe in one-day-a-year gratitude. Really, should not all who profess Christianity encourage a spontaneous spirit of thanksgiving—from the heart—the year around?"
This argument doesn't work as well with Canadians because they celebrate Thanksgiving in October, which isn't the beginning of the Christmas season.