Here's an easy way of seeing it. It's a plot of pressures on the bodywork. Where the solid line is above the body, the pressures are low. Where the line is below the bodywork, the pressures are high. Note also the direction of the arrows that shows how those pressures are developing forces (ie they are vectors).
We can see that the attached flow over the car (right up to the separation that occurs at the base of the hatch) creates a lot of lift. (Only at the base of the windscreen, and a tiny bit at the trailing edge of the hatch, is that pressure creating any downwards force.)
You can also see from the magnitude of the lift forces that it's very hard to offset all of them with anything on top of the car. Therefore, we need to work under the car, where we have a large area - and so even lowering pressures a bit will have a big impact. (Obviously, this diagram doesn't show any of the pressures under the car - in those days, the undercar was basically ignored.)
Finally, note the magnitude of the lift pressures compared with the wake pressures - lift forces can be very large.