I don’t know for sure but I suspect they started with 5V because the IQ Panel was based on the Android platform and tablets run on 5V (USB power).
Then they realized 5V is too low with too much voltage drop and increasing to 7V was the easiest way to solve that without completely changing the platform.
Then they finally realized 12V is better so they used that for the IQ Remote PowerG.
I think the next IQ Panel will be 12V. They should have used 12V across the board from the start.