Historically, Thanksgiving is the US equivalent to the Jewish Sukkot – one of the harvest holidays where we celebrate the bountiful crop. In that respect, it’s based on the change of seasons, and the earth’s agricultural renewal.
July 4, while important, has a history of only 240 years, and is based on some people signing a piece of paper. Although important for the country, it doesn’t have the same anthropological and tribal significance as the harvest holiday.
Also, Thanksgiving is always a Thursday, and many people make it a 4-day weekend, meaning that travel is enabled and families find it easier to get together.
In the end, it’s about the money. If stores could figure out a way to be open on TG Day and not offend people, they would,