because you need to have the cylinders synchronized so that a specific one is on the power stroke while specific other ones are on their compression stroke (which requires that other's power stroke). I'm sure you could have it set up with many different firing orders, but I'm assuming that it would throw off the crank shaft's balance quite a bit compared to this commonly used sequence.
153624
153624
153624
153624
153624
153624
STRAIGHT 6 cylinder - Ford , GM , Chrysler etc.
1-5-3-6-2-4
firing order 153624.......rotation is clockwise
the Standard firing order on most 6 cylinder in line engines is 153624
firing order is 153624. #1 cylinder is in the front of the motor.Clock wise rotation on the distributor.Hope this helps,Good luck.
I rebuilt one 25 years ago. Its 153624.