Because of its inventive use of blue notes (pentatonic scales), call-and-response, improvisation, polyrhythms, syncopation and the swung note of ragtime and the fact that it also utilised original tribal and folk/cakewalk music and incorporated music from religious hymns combined with Eurpoean folk and dance music. Many early black musicians used their musical ethnicity to play European instruments (violin etc). It was different, it was a modern blend at the turn of the 19thC and popular because of it.
Don�t confuse modern contemporary �Belgian� jazz with the old, ragtime, Swing and New Orleans jazz. In doing so, you serve the origins a great injustice, since this modern malarkey sounds like twenty people playing together at the same time but each playing different tunes in a different key in a different time signature.