Because, unless you’re driving a forklift, the point of a vehicle’s rotation is in line with the rear wheels, meaning you can take turns at a much more acute angle when reversing than going forwards. Which makes backing into spaces much easier.

Notice that most of the half-assed parking jobs you see are generally people who have driven forward and left the car parked at a diagonal half out of the space, because getting the vehicle lined up in that situation is more difficult.