Why did the Supreme Court rule that the federal child labor laws passed by Congress were unconstitutional?

Because it overstepped the purpose of the government's powers to regulate interstate comerce.

Explanation:

After becoming president in 1934, Lázaro Cárdenas tried to fulfill the ideals of the constitution by taking the following actions:

1) nationalizing oil and railroad industries
2) improving education in rural areas
3) supporting trade initiatives
4) redistributing land to the peasants
5) establishing a dominant political party