Here's why:
* Sentences have a subject and a verb. This phrase describes an action (dancing) but doesn't tell us who or what is doing it.
* Sentences express a complete thought. This phrase is simply a description.
To make it a sentence, you could add a subject:
* She playfully danced in the snow.
* The children playfully danced in the snow.
Let me know if you'd like to explore more about sentences and phrases!