why onreadystatechange doesn't follow the camelCase convention?

Question

例如：

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
          document.getElementById('ajax').innerHTML = xhr.responseText;
      }
};

notice xhr.onreadystatechange is not camelCased but xhr.responseText is. Is there a reason for this that I've missed?
还有诸如：onmousedown/onmouseup 等等，都不是驼峰命名的