I heard that this tribe does not have alphabets to write. But instead using ABC, why they using 한 글?
Actually, they don't use Korean characters (any more). It was a misguided attempt by a group of Korean activists, which eventually fizzled out, as far as I know.
While it's unfortunate that the plan did not follow through, the logic for using 한글 over the Roman alphabet can be as follows: 1) more exact phonetics compared to the Roman alphabet. 한글 has a (more or less) 1:1 correlation between character and pronunciation, as opposed to the multiple possibilities of the Roman alphabet (Ex: 'A' can be a long 'A' as in 'Ant' but also a short 'A' as in 'Apple') 2) easy to learn. The Hangeul follows a visual mapping of the mouth that linguists have lauded as scientific and logical. 3) Hangeul is a syllable-based alphabet, meaning that it clearly depicts distinctions from one syllable to the next (i.e. each 'block' is a syllable, while in English it is impossible to tell from just the written form)
If for all these reasons Hangeul better suits the Cia cia tribe's language, it makes sense to choose it over the Roman alphabet. But of course, other factors could still weigh in on the decision, such as globalization, politics, etc.