Why? Because they rode horses, wore armor and had guns.
Going up against a stone age people who didn't even work in metals or the wheel.
The Aztecs may have been as advanced as the ancient Romans, but the ancient Romans by 1492 in European history were well, ancient history.
By then they had become more advanced than The Romans, with things like the Compass, Eyeglasses and Guns.
“The Aztecs may have been as advanced as the ancient Romans,”
Not even close.