Can anyone explain why we need unequal length wishbones on the front.

It stops the roll centre moving about too much. It's a feature of all double wishbone suspension systems, infact americans call double wishbone suspension "Short-Long-Arm" suspension in reference to the top wishbone been shorter than the lnger lower one

If the top link is shorter (which it is) then as the wheel rises the top of the wheel will be pulled in more than the bottom, and negative camber will result.
When a car is cornering it will tend to lean toward the outside of the corner, so the wheel rises relative to the chassis, and the resultant negative camber helps to keep the most tread on the road

John