In still air, a helium filled balloon rises up to a certain height and then stops rising, why?

  

The upthrust on a body due to a fluid depends on the weight of displaced fluid. In the lower height the atmospheric pressure is high. 

so the balloon displaces the sufficient weight of air to get the upthrust to lift it up. As it rises up, atmospheric pressure gradually decreases. 

So thea of displaced air by the same volume of balloon is low. Therefore, the upthrust of air is not sufficient to lift up the helium filled balloon. So, it stops rising after a certain height.