It seems that it has to do with the use and application of vowels which differ in various languages:
Quote:
Vowel signs were originally not used in Semitic alphabets. In the earlier West Semitic family of scripts (Phoenician, Hebrew, Moabite etc.), a letter always stood for a consonant in association with an unspecified vowel or no vowel. This did not reduce legibility because words in Semitic languages are based on consonant roots that make meaning clear with only the consonants present, and vowels are clear from context. By contrast, Greek is an Indo-European language, and thus differences in vowels make for vast differences in meanings. Thus, Greek grammarians divided the letters into two categories, vowels and consonants, which had to be accompanied by vowels to create a pronounceable unit.
http://en.wikipedia.org/wiki/Greek_alphabet
For comparison, here's a link to the use of vowels in Semitic alphabet, which is notorious for it's lack of written vowels - for instance, famous JHWH which, of course, is forbidden to pronounce, otherwise you might get stoned.
http://www.hebrew4christians.com/Gramma ... ction.html