Why would the No Child Left Behind emphasis on math and reading leave less time to learn history?

I'm reading this Wall Street Journal article about new history history test scores:

The overall lackluster performance is certain to revive the debate about whether history and other subjects, such as science and art, are being pushed out of the curriculum because of the focus on math and reading demanded under the No Child Left Behind federal education law. The federal law mandates that students be tested in math and reading.

Why is this even a conflict? To learn reading,  you need reading material! Make the reading material nonfiction works about history. Problem solved.

I have discussed this issue before, and I know people have a problem with it, but schoolchildren don't need to read fiction to learn how to read. Let them choose their own fiction books for reading outside of school.