Because, helium is even lighter than air. A liter of air weighs about 1.25 grams, but a liter of helium weighs only 0.18 grams. A balloon that's 1 foot holds about half a cubic foot of helium, which is the same as 14 liters. This means the balloon will weigh about half an ounce less than the same sized balloon filled with air. With that being said, it being lighter than air makes it go up. Hope this explanation helped:)
