Not really understanding expansion.
Your subfloor expands and contracts too. When it does it takes the hardwood with it, nails and all. If you were to hard nail the quarter-round to the floor, and the floor contracted, it would pull away from the wall. If it were nailed to both wall and floor, it would likely split. Expansion and contraction can literally exert tons of force on a structure. That is why building techniques have evolved that compensate for those forces. Allowing the floor to float under that molding is one of those techniques.