Because the flag is a material item and we are a material society.

Almost anyone can buy a flag. And if you have more money than your neighbor, you can buy an even bigger flag. Then you can bow down in front of the flag and sacrifice your first-born.

But the Constitution?

That is an abstract document that most people are not familiar with and are too embarrassed to admit it. To them, it's just another random piece of document from a period in history where the men wore wigs and stockings.

Most people couldn't tell you the difference between the Constitution and the Declaration of Independence. Most people have no clue when their Constitutional rights are being violated.