It's called Thanksgiving for many reasons

Washington celebrating Thanksgiving at Mt. Vernon
When President George Washington issued the first Thanksgiving Proclamation under the U.S. Constitution on October 3, 1789, he was continuing an age-old tradition but adding a uniquely nationalistic slant. The United States had recently emerged victorious from a long and bitter war with the greatest power in the world, and its leaders were deeply immersed in creating a bold system of government that would ensure and protect the rights of its citizens. It was indeed a propitious time to give thanks. Continue to full article.