The reason is loss. I.e. loss of voltage. When you need a 3 kW starter, there will be a high current flowing through it, and that current flows through the battery and cables as well. At 12 volt, the current will be 250 Amps. At 24 V only 125 A. The higher the current, the higher the voltage-loss through the cables and terminals. If at 250 A, the voltage loss is 2 V, then at 125 A the loss is only 1 V. Than factor in that 2 V is 16 % of 12 V, while 1 V is only 4 % of 24 V. That means the loss of power in the 12 V starter is four times that of the 24 V starter.
This is slightly simplified, the difference can be even bigger. But as this is all theoretical, and not considering adaptations done to rectify the problem, it's good enough for highlighting the problem.
So, in the end, with all the hassle of change-over relay etc, it is better and more reliable to design the 12 V starter for a bit lower voltage (to compensate the loss) and then use thick and short cables.