The answer is pretty simple: the helium leaks out. Helium is a very small atom and latex is pretty porous at the scale of a helium atom (if you ever go looking to buy balloons, you will see that there are "helium-grade" balloons, which try to be thicker and less porous).


See How Helium Balloons Work for complete details on the weight that a helium balloon can lift!

Here are some interesting links: