It is because of the hydrogen, but it's not really to do with the mass, but rather because a "collision"/interaction is much more likely. For instance, ^58^Ni has the same likelihood to interact as hydrogen, while deuterium has only a fifth. ^135^Xe meanwhile has some millions of times the absorption power of hydrogen and it's quite a lot heavier. It's just not very practical as an absorber.

Hydrogen's cool man. It's just like, one little proton out there in the universe, with his Jiminy Cricket Electron buddy circling around all times, making sure he stays balanced. An unfathomable number of them, more than anything else. All exactly the same yet repelled by each other, like speed dating at comic-con.