Let me try a 3 year old version:

-Trees with big leaves go through cycles every year where they lose their leaves in the fall and grow new ones in the spring. Trees with skinny needles instead of big leaves don’t do this.

-The winter weather is too cold for the leaves, so the tree doesn’t need leaves in the winter

-The tree makes the leaves drop off because by the time winter is over, the leaves would be too damaged to use and the tree would need to make new leaves anyway.