Simply because while both gases are lighter than air, Hydrogen is extremely volatile and liable to ignite whereas Helium is inert (I think !)
Helium doesn't burn. It's a noble gas. Google it
Helium and hydrogen are both very light gases however helium is completely unreactive (inert)
I don't know.....^_^my teacher said that because if they use hydrogen,it will fly up to the air and we also can fly.so,its very dangerous if you use hydrogen to the balloons.