Answer:
because then the Bill of Rights would cover both state and federal law
Step-by-step explanation:
In United States law, the Establishment Clause[1] of the First Amendment to the United States Constitution, together with that Amendment's Free Exercise Clause, form the constitutional right of freedom of religion. The relevant constitutional text is:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof...
The Establishment Clause acts as a double security, for its aim is as well the prevention of religious control over government as the prevention of political control over religion.[2] Under it the federal government of the United States as well as the governments of all U.S. states and U.S. territories are prohibited from establishing or sponsoring religion.[2]
The clause was based on a number of precedents, including the Constitutions of Clarendon, the Bill of Rights 1689, and the Pennsylvania and New Jersey colonial constitutions. An initial draft by John Dickinson was prepared in conjunction with his drafting the Articles of Confederation. In 1789, then-congressman James Madison prepared another draft which, following discussion and debate in the First Congress, would become part of the text of the First Amendment of the Bill of Rights. The second half of the Establishment Clause includes the Free Exercise Clause, which allows individual citizens freedom from governmental interference in both private and public religious affairs.