in 1870, the state of Tennessee adopted a new constitution. Which is the BEST explanation for why this happened?

The best explanation to adopt a new Constitution in Tennessee is that it was a response to the requirement for all ex-Confederate states to adopt new constitutions expressly prohibiting slavery. In that sense, General Assembly, on November 15, 1869, called for an election to determine if a constitutional convention should be called to amend or replace the 1835 Constitution and to elect delegates to that convention. Finally, the voters decided for the convention and the new Constitution was approved in March 1870.