I think it's because Copt is a proper noun that would be written with a capital letter. Proper nouns are not allowed in Scrabble. This is also true for Catholic and Protestant when referring to the churches, but those words can also be used uncapitalised with (slightly) different meanings - one may have catholic tastes, or someone who protests can be called a protestant.
Anyway, any takers for ENTER -> LEAVE ? It's an interesting one because wherever there is a vowel in one of the words, there is a consonant in the other one.