Or, because it does drink snow-water, but it does not drink dew-water, it is written "From the rain of the heavens you will drink water" — even dew-water.
And just as dew descends for blessing, so, rain. And thus is it written (Bereshith 27:28) "And may G-d give you of the dew of the heavens," and (Devarim 32:2) "My teaching (Torah) shall drip as the rain," and (Hosea 14:6) "I shall be like the dew to Israel," and (Michah 5:6) "And the remnant of Jacob shall be in the midst of many peoples like dew from the L-rd, like the droplets upon the grass."