Snow and hail are refused because eating them destroys water-bodied beings for no need: water itself is necessary to life, and frozen water is not.
The commentary's distinction is exact. Water is apkāya, a body of one-sensed beings, and a layperson takes it because there is no living without it; snow (hima) and hail (karaka) are the same beings with the need removed, so the harm is gratuitous. The modern practice extends the rule to ice, which the medieval verse could not name. Earth is the third of the three raw elements on the plate.
The twenty-two, laid by their reasons - Snow highlighted
Hail, karaka, stands in the verse two places after snow and for the same reason: water-bodied beings, and no need.
The verse has hima-visa-karage, snow, poison, hail, and poison sits between the two frozen waters for no reason but metre. The snow entry carries the reason; the plate puts the two side by side where the verse does not.