Kapilavastu, the kingdom where Siddhartha was a prince was in fact a small mountainous and insignificant place and by becoming the king of such a small backward, tribal kingdom Buddha would have reduced himself to a very insignificant position. Remember that as a religious leader he had Bimbisara and Prasenajit, kings of Magadha and Kosala, two of the most powerful kingdoms in India, as his devotees, ready to carryout any of his orders. There were several other kings and princes, rich merchants and traders among his followers.
Further abandoning the simple robes of a monk and donning the royal dress of a king would have made him an object of ridicule and contempt. In ancient India, the priests and monks, received more respect and adulation than royals, generals and rich merchants. There are several stories from ancient period which tell about kings and princes trembling in front of monks and falling at their feet, if they are displeased even slightly. From the earliest Vedic Period down to the present, Brahmin, the priestly caste, is considered to be the highest in Hinduism. Buddha, though his mission was to challenge Brahminical authority, he was considered to be someone who possessed such an authority as a preacher.