I think it has to do with design. They wanted to make the two top rows have the same amount of lights. Using 6 would mean one row of 6 lights and one of 4. 5^2

I think it has to do with design. They wanted to make the two top rows have the same amount of lights. Using 6 would mean one row of 6 lights and one of 4. 5^2 is the smallest square that provides enough values to fit 24 hours.