Here's why:

* Helium: Helium is a very light gas and wouldn't be found in significant amounts within solid asteroids.

* Hydrocarbons: While some asteroids might contain trace amounts of hydrocarbons, it's not a primary component.

* Hydrogen: While hydrogen is a key component of water, it's not found in large amounts in asteroids in its elemental form.

Water ice is a common constituent of many asteroids, particularly those in the outer asteroid belt. The presence of water ice in asteroids has been confirmed by various spacecraft missions, including NASA's Dawn mission to Ceres.