You don't need so many amps in a 48v system to get the same power. So the same labour is less demanding on the battery, and it runs down more efficiently.

A 15ah 48v battery is the same as a 20ah 36v battery in cell count and available power. Just configured differently. The 48v may last longer depending how it's used. It also has the ability to give higher power in short bursts, and a higher top speed, if the controller allows.