This suggests it is because they wanted to limit frontal area and rolling resistance to an extreme level, even limiting the effect of the steered wheels.
Quote:
PAC-Car II is equipped with 3 wheels; the single rear wheel is powered and steered, and the front wheels have a camber angle of -8°.
This solution allows the reduction of the frontal surface area because the room needed to steer the wheels is not needed. Some experiments on a test bench have shown that this camber angle does not provide too much rolling resistance.
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So they put the steered wheel at the rear where it would be shielded by the bodyshape, and the two at the front in a fixed position to minimise RR. It did its record breaking at 18 MPH though.