Note: Although some letters may be more frequent, the letters only fit into the common features of country names.
This is a combination of several factors which makes up the most common name possible. It includes whether the most occurrences per letter are vowels or consonants (I.e. vowels only comprise 32-38 of first letters of countries). Also, even if there is a more popular letter for a certain space, if it is not the write type of letter (vowel/consonant) it cannot be accepted. Enjoy :)