Because ice crystals were designed by God to form in a certain way based on temperaturate, moisture content, particulate matter, etc.
Ice crystals were designed NOT to occur in other ways. For examples, if the temperature is 120 degrees than ice crystals will never occur.
There's nothing random about it. If we had the eyes to see we could follow the snowflake back way earlier than its physical origin. We could follow it right to the mind of God.