Question

Why does the ozone layer only forms in the stratosphere?

Answer 1

ozone forms as a result of the uv rays splitting up the O2 molecules into single atoms of  oxygen

O2----> O+O

this independent oxygen molecules combines with O2 molecules and forms O3 (ozone).
This formed ozone prevents the uv rays to enter the earth i.e.. it does not allow it to pass beyond it. That is why ozone is only found in stratosphere.
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Answer 2

Because all the weather phenomenon such as rain,hail storm,etc.occur in the stratosphere. Thus the oxygen molecules present in the air combine together to form ozone layer. 
Ozone layer's chemical name is O3.