The only arguement to choose a higher voltage PSU is just that a lower output current is needed making it cheaper.
But because ther is more to run on 12v like tire heaters, pit light etc the choice for a high power 12v PSU is much wiser.

The X6 is a bit special. There are some situations where it does need a higher voltage PSU to output max power. But not for 2 cell.

All the high-power iChargers are like that and probably the other brands too. They have derated output power with 12V supply. I have a six year old i306B and it is the same situation. It's not complicated. At 12V the input current max is exceed before the output power max.

if you knew how 12 volts DC is boosted to a higher DC voltage.. its costly.. and sucks lots of amps..

ac is just Different and much easier to up boost..