How did Abu Dhabi become a capitol?

Previous answer not completely true, but most.

The plan after the 7 Emirates established the unity as a nation was to make every emirate the capital and its sheikh the ruler of the UAE. When Sheikh Zayed (Abu Dhabi)'s reign was at it's end around 1976 leading to Sheikh Rashid (Dubai) to become the next. Sheikh Rashid rejected his turn and announced to declare Sheikh Zayed the official ruler and the Emirate of Abu Dhabi as the capital because Sheikh Rashid did it out of kindness and chose the best for his country in making this decision.

I too agree that Sheikh Rashid has done a fine job.

FYI, the first union formed between Abu Dhabi & Dubai -the both were tribes of Bani Yas- in 1968, the invitations to other 7 were sent: Sharjah, Ajman, Umm Al Quwain, Ras Al Khaimah, Fujairah, Qatar, and Bahrain. Qatar and Bahrain later rejected and declared independence from the British colony. The emirates discluding Ras Al Khaimah formed the UAE (releasing itself from the British colony). At 1972, Ras Al Khaimah broke its way out of the British colony. Ras Al Khaimah was the most affected city in terms of wars in those days -it was the major place.

There is this amazing documentary video on Sheikh Zayed: http://ca.youtube.com/watch?v=Sck1nXyseC...

in 1968 sh.Zayed of Abu Dhabi and sh.Rashid of Dubai meet to unit the tow emirates. and laiter they started sending invitations to join the unity to 7 emirates whec were "Bahrain, Qatar, Sharjah, Ajman, Um alquwain, Fujairah, And Ras alkhaima". in 1971 4 of them agreed to join which were Sharjah, Ajman, Um alqwain, And Fujairah" and they met in Dubai do declaer the unity of the 6 emirates and according to the constitution each ruler vote for a president and vice president. so, because abudhabi and Dubai are the founder of the unity and because sh.zayed and sh.rashid are more powered the were chosen. and then for every 5 years the rulers meet to vote and it been the customs that the ruler of Abudhabi is the president, and the ruler of Dubai is the vice president. anyway, in 1971 when the 6 rulers decided that the president is sh.Zayed Abudhabi was chosen as the capital accordingly. in 1973 ras alkhaima joined the UAE and they became 7.