The pressure pattern over the wing goes from high at the leading edge to low at the point of maximum camber and then back to high again at the trailing edge. So, from the point of max camber backwards the airflow is moving from a low pressure to a high pressure region - called an adverse pressure gradient. The air would not do this unless it was being pushed by some other factor and this factor is the kinetic energy of the moving air. The boundary layer has less KE than the freestream air and gradually slows down. When it stops or reverses it breaks away from the surface, or separates.

As the laminar type of boundary layer has less KE than the turbulent type it will slow down quicker and break away earlier. So the laminar BL has less KE and breaks away earlier. The laminar layer, however, causes less drag.

To try for the highest possible lift at high angles of attack you need to keep the airflow attached as long as possible and this means having a high KE boundary layer. Various high lift devices are there simply to re-energise a BL that is slowing down. Never mind the drag, it is lift we want at the stall.