The most accurate response to why early American political leaders wanted a combination of democratic and republican structures in the U.S. government is:
They believed it was important to allow people some voice in their government, but wanted to ensure the right people maintained power.
This response reflects the Founding Fathers' desire to create a system that allowed for popular input through democratic processes, while also instituting checks and balances through republican elements to prevent potential mob rule or the dominance of factions.