What determines the commonality of a nova versus the rarity of a supernova?

Nova commonality relates to the stable binary transfer system, while supernova rarity ties to complete stellar destruction.

The frequency of these two events is dictated by the underlying physics of their occurrence. A nova's commonality is tied to the stability and recurrence of the binary accretion process; as long as the white dwarf and its companion exist and material transfer continues, future eruptions are statistically probable, even if separated by centuries. In contrast, a supernova is rare because it signifies the end state of stellar evolution—either the complete exhaustion of a massive star's core fuel or the catastrophic mass overload of a white dwarf. Since complete stellar destruction or fundamental restructuring is a much less frequent outcome than a surface flare-up, the supernova remains significantly rarer than the nova.