The ventral radiator placement in many WW2 fighters was supposed to be less draggy than underwing radiators afaik? So why wasn't that changed with the Me 109?
Examples were the series 5 Italian fighters, the Ki-61 and the Yaks.
Up until the Me 109D the Jumo 210 engined Me 109 versions had a chin ventral radiator. The 109E introduced the twin wing radiators but the oil cooler stayed in the chin position. The Me 109E radiators were sophisticated and recovered heat as thrust. A split flap arrangement controlled airflow with the normal split flap forming the lower part of the nozzle and the upper being it's own flap. in the Me 109F a boundary layer splitter was added.
One obvious reason for not having a ventral radiator is not having that position available for a fuel drop tank or bomb.