Since Aladdin who is a magi, as one who deem one worth to be king choose Alibaba who he believe strongly that he'll be a king candidate of a future nation/country. Hope this helps.
Oh okay, thabk you very much ^^
Also (apart from being the king... king candidate, actually, Aladdin chose), he was a prince from Balbadd. A bastard one, but the king recognised him and wanted him to be his sucessor. So, by heritage he IS a king... Just not crowned because his kingdom was conquered before.
Can anyone explain to me why they all refer do Alibaba as "king"? I mean I like him very much, but it just confuses me a little :D