Also (from the article) the state of the body - that it had wounds consistent with descriptions of Richard's death, that the wounds were confirmed to have been

Also (from the article) the state of the body - that it had wounds consistent with descriptions of Richard's death, that the wounds were confirmed to have been inflicted pre-mortem, and that the other general characteristics of the skeleton were consistent with our knowledge of Richard. The confluence of factors is what allows such a high probability to be ascribed to the hypothesis.