Why should one choose the maximum as reference for power-to-decibel conversion?
dB is a power ratio, so when we see units in dB we are seeing units on a relative scale. The reason that a 0 dB reference is so common is because this is simply normalizing the number scale to 1. $10Log(1) = 0$ dB

This is similar to using 100% (1) instead of absolute numbers.