Simply because while both gases are lighter than air, Hydrogen is extremely volatile and liable to ignite whereas Helium is inert (I think !)
Why Is Helium Used To Fill Balloons Instead Of Hydrogen?
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.