We get different kinds of snowflakes depending on the temperature and humidity. The vertical axis here shows the humidity in grams of water per cubic meter. The curve shows when the air is saturated: it can't hold any more water above that curve. I'm no expert, but I bet snowflakes gradually shrink below this curve due to sublimation: ice turning to water vapor. As you can see, there are no fancy snowflakes below this curve. I don't know why this curve has a peak at -15 °C.
While this chart raises lots of questions, and I don't know the answers, I know the story is even more complicated than this, because as snowflakes fall or get blown upwards by updrafts, they encounter air with changing temperature and humidity! Their ultimate shape is a record of their whole history.
I got this graph from the Alaska Lake Ice and Snow Observatory Network: