
Apart from that there are several minor reasons:
— phonetic transcription varies according to dialectic details of language (e.g. RP–General American—Cockney. This makes the phonetic alphabet imprecise and not universal;
—English letter alphabet has 26 variables, while English RP phonetic alphabet consists of 24 consonants and 23 vowels (plus 6 additional sounds used in foreign words). I picked the smaller alphabet, as the higher the numer of variables in the code, the more noises needs to be used / overlapped, which leads to decreasing respectively the distinctiveness / clarity of produced sounds.
—letter alphabet is commonly known unlike the phonetic IPA alphabet which is rather a specialist knowledge.