Here's why:
* Asteroids are rocky bodies: They are primarily composed of rock, metal, and sometimes ice.
* Hydrogen is a light gas: It easily escapes into space, especially in the relatively weak gravity of asteroids.
* Solar wind: The constant stream of charged particles from the Sun would quickly sweep away any hydrogen that might be present near an asteroid.
Exceptions:
* Some comets have hydrogen clouds: Comets are made of ice and dust, and when they get close to the Sun, the ice vaporizes, releasing gas and dust, including hydrogen, forming a coma and a tail.
* Tiny amounts of hydrogen might be trapped: Some asteroids may contain small amounts of hydrogen trapped within their minerals, but this is not a significant amount and wouldn't form a visible cloud.
In summary: Asteroids are not known to be surrounded by hydrogen clouds. The conditions in space don't allow for a hydrogen cloud to persist around these rocky bodies.
