I don't want to revive dead threads but this is the information I have gathered from some research into Buddha's origins. First Keay is correct in his statement that Buddha was born in a tribal republic where the most influential family were the Shakyas. Suddhodhana was the elected head of the republic (ganarajya, incidentally official Hindi name for Republic of India is Bharat Ganarajya.). Needless to say Suddhodhana was a first among equals.
This era was the rise of kingdoms and the downfall of the tribal republics (which closely resembled the early Aryan societies). We can see this by the expansion of the kingdom of Magadha. King Bimbisara started this trend and it was followed by his son Ajatashatru who defeated the powerful Licchavi clan of the Vaji confederacy and of course the anhiliation of Buddha's own clan by the kingdom of Koshala. The Arthashaastra, the political treatise written by Chanakya, mentions how to eliminate tribal republics by using propaganda, assassinations, and other morally questionable things. Ironically Kalinga that Emperor Ashoka conquered was also a tribal republic.
As we know from history, the thing that transformed Buddha's Dhamma from just another sect of the Gangetic plains to a worldwide religion is state support. Massive state support from Emperor Ashoka. Ashoka is said to have built over 80,000 viharas and propagated Dhamma across Asia. Ever since then, due to its highly organized community and its large centralized structures: Buddhism has been dependent on state support for its survival. More specifically imperial support. Due to this, maybe later Buddhists liked to imagine Shakyamuni as being an actual prince.