Because it is truly and uniquely Slavic?

Hm, same could be said for Cyrillic, altough Cyrillic is adaptation of Greek alphabet, letters for Glagolica were taken from other alphabets, Greek, Syrian, Armenian, Coptic etc.

IMO, it should be simplified, though.

Acctually, it could be applied for any Slavic language with not to much effort, but it would take enormous costs for switching with no real gain.