Why did some federalists oppose adding a bill of rights to the constitution?

the states' already protected citizens' rights

Explanation:

personal guarantees and freedoms that the government cannot abridge (curtail) by law, constitution or judicial interpretation.

Bill of right gurantees the following

Guarantees the freedom of religion, speech, press, assembly and the right to petition government ... Guarantees the right to a speedy, impartial public trial in criminal cases with counsel and the right to cross examine. 7th Amendment. Guarantees the right to jury trial in civil suits involving $20.00 or more.