This is a serious question...If States are subject to the Federal Bill of Rights, why have any state constitutions at all?
Otherwise, Virginia may have protections against unreasonable searches and seizures and Georgia might not. Or North Carolina would have the right to keep and bear arms and Connecticut might not.
The primary reason for a government is to protect it's citizen's rights. Everything else is pursuant to that purpose.
I would guess they wanted to make sure that the rights of all citizens were protected from all governments. The states constitutions were to lay out the form and function of their own governments, provided it was not in violation of the rights protected by the Bill of Rights.