Why is γράμμα used here and not επιστολή ?
Both are possible.
Remember that they have different genders: το γράμμα is neuter, η επιστολή is feminine. (The adjective also has to agree in gender with the noun.)
That said, το γράμμα is more common than η επιστολή, in my experience.
Thank you. I have only heard το γράμμα . Never η επιστολή