User blog comment:WeissSchnee/Beacon and Signal/@comment-24038207-20130721051520

Most likely it has to do with their combat skills. Glynda did have a recording of Ruby fighting, and Ozpin might've been reviewing that recording outside of the room. Yang obviously worked to get into Beacon through hard work, but Ozpin, considering that he saw one of two(possibly the only two if there's no more mystery) people that are able to use the Crescent Rose efficiently and effectively. Knowing this, Ozpin most likely wanted Ruby to come to Beacon.