Question

All snowflakes have six sides but why do every snowflake has a unique shape.

Answer 1

not all snowflakes have six sides. some have less, and some have more. no snowflake is alike because of the water molecules it's made of and the environment/process it's made in/through. the molecules of a snowflake develop at different rates and patterns, so it's almost physically possible (without some extremely controlled environment) to make/find two snowflakes that are the same. 

Answer 2

All snow flakes don't have 6 sides it is only in imagination