if you look at a standard diagram of a spinal cord and nerve
you will see that a nerve root is either all sensory or motor
and cutaneous is gonna be mixed
or if you dont like that....
nerve roots follow the dermatomes - look at a dermatome diagram a few pages on
whereas named cutaneous nn follow the distribution of the named nerve.
or......nerve root contains info from other sensory inputs - muscle spindles and joint position
where as cutaneous only have sensation, pain, heat etc.