Standing with 2 feet, apart, looking at the lightning, the voltage both feet see is the same, so no current though the body. Turning 90°, the feet now in line with the strike location, they see the maximum voltage difference and current goes through the body.

Think of 2 series resistors and of those 2 in parallel (as the ground before and after the person) and one that connects the 2 parallel ones in between the 2 series ones (body). -- R -- ,-- R -- --| R | -- -- R --'-- R --

Only if the 2 R top and bottom are different will current flow through the middle resistor (body).