There are several reasons for this: There is a war going on and morale among the population and shinobi should be kept high. His brother died gloriously in battle and in order to comply with his wish, the raikage promoted him. In times of peace, things would be different, but there is a war going on and the higher-ranking shinobi are almost all on the move. Besides, Yoroi is very talented and it is only a matter of time before he reaches the level of his new rank. Because of the memory loss, he had already been given several months' time off and his skills were tested in a test fight. As he had performed quite well, his rank was retained, otherwise he would have been demoted again. Finally, it is also a measure by the raikage to be able to put together a team of genin. They would actually want to appoint a Jonin as team leader, but as I said, there is a war going on.