May be when Melkor created the Trolls he did not know which ability sunlight would have. Trolls were most likely created during Melkors time in Utumno. Since he did not listen too much to the song Melkor would know of the coming of the sun may be but may be not of exactly what sunlight would mean for his creation.
Even if we take a round earth version of the mythology we can argue that Trolls were created during the long darkness that Melkor produced by clouds.
Respectfully
Findegil
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