Two reasons come to mind. First, Thanksgiving involves no "presents." We don't spend madly money we don't have to stock up on "Thanksgiving presents," instead we pause to recognize one of our greatest gifts, the hand of divine providence in the creation of our nation. But we no longer attach value to God, so we no longer value His gifts.
This leads to the second reason Thanksgiving has become so neglected, which is the revisionist history being propagandized to our children attacking the traditional Thanksgiving story as nothing but make-believe folklore. The Pilgrims are now lumped into the stereotype of evil, racist, white male Europeans who came to destroy the Indians (ops, sorry there I go again, I mean "Native Americans"). The reality is that the story most of us has learned, while romanticized in parts, is generally accurate. In fact, the Pilgrims and the Indians signed a peace treaty which lasted for over 50 years. The war that broke the treaty was actually a European war that spilled over into the colonies and sucked in both colonists and Indians.
So today, let us pause and give thanks for the gifts we enjoy. As George Washington said:
"It is the duty of all Nations to acknowledge the providence of Almighty God, to obey His will, to be grateful for His benefits, and humbly to implore His protection and favors."
–George Washington, Thanksgiving Proclamation, 3 October 1789