Because the wing loading is higher and therefore the stall AoA is lower (remember - THERE IS NO SUCH THING AS STALL SPEED!).
But there is such a thing as a maneuvering speed, which is what the OP is all about. Actually reading the question before answering is a useful exercise.
Add to that the fact that such a statement is utter bollocks, the stall AoA is constant for a given profile, irrespective of wing loading.
As for the original question, it's just like Captain Andrew said. With a higher wing loading you reach stall before you overstress the plane from a higher (i.e. greater speed) starting point than at a lower wing loading, exactly because critical AoA is constant but airspeed for a given AoA is proportional to the square root of current wing loading.
Ciao,
Dg800