The air behind a car is generally low pressure + low velocity. So the total pressure of the the air is also low because total pressure is static + dynamic pressure. Total pressure could be said to be analogous to total energy where your dynamic pressure is analogous to the kinetic energy and static pressure is analogous to your potential energy.

So basically the air in the wake of a car has low energy. This low energy air can't generate the same amount of downforce high energy freestream air can. So on straights , downforce generated is low so your drag is also low. On turns , you need a lot of downforce . So this wake now becomes dirty air.

Dirty air and slipstream are essentially the same thing. It's just that they call it dirty air during turns and slipstream during straights.