Because the wing loading is higher and therefore the stall AoA is lower (remember - THERE IS NO SUCH THING AS STALL SPEED!).
Not sure about any of this statement - IIRC the stall AoA is actually the same no matter what the weight is. With a higher wing loading due to higher mass the 1G (and for that matter that at any other set G) stall will occur at a higher speed.
There is such a thing as a stall speed, but it does need to be defined!