Because there is now more airflow from the front, causing the area of reverse flow to increase (move outboard), taking with it the stalled area, the driving area, and the driven area.
Go fast enough, and a few things can happen :
(a) the area producing lift gets too small to support the weight of the aircraft;
(b) retreating blade stall comes along to stuff everything up, and
(c) the drag on the blade slows the RRPM down and it all turns to worms.