liquid will always take path of least resistance, the radiator is a very convoluted place where the liquid needs to pass through small channels, it can flow easily through the big passages of the block and head. That is why cars with a positive bypass thermostat (like this mitsubishi diesel) will overheat badly if the thermostat is missing.
in simple terms - when the thermostat opens up, it blocks off the bypass port. This means the liquid has to pass through the radiator as it has nowhere else to go.