问为什么jQuery不能直接操作SVG元素的类？

EN

(function ($) {
    var element,
        jQueryFunctions = {
            addClass: $.fn.addClass
        };

    var addClass = function (
        elements,
        value) {
        var proceed = typeof (value) === "string" && value;

        if (!proceed) {
            return this;
        }

        for (var i = 0, j, l = elements.length, element, klasses = (value || "").match(/\S+/g) || [], klass, currentValue, current, finalValue; element = elements[i], i < l; i++) {
            currentValue = element.getAttribute && element.getAttribute("class") || "",
            current = element.nodeType === 1 && (" " + currentValue + " ").replace(/[\t\r\n\f]/g, " ");

            if (!current) {
                return;
            }

            j = 0;

            while (klass = klasses[j++]) {
                if (current.indexOf(" " + klass + " ") < 0) {
                    current += klass + " ";
                }

                finalValue = jQuery.trim(current);

                if (currentValue !== finalValue) {
                    element.setAttribute("class", finalValue);
                }
            }
        }
    };

    var hasSvgNodes = function (
        elements) {
        var returnValue = true;

        for (var i = 0, l = elements.length, element; element = elements[i], i < l; i++) {
            returnValue = returnValue && (element.nodeName === "svg");
        }

        return returnValue;
    };

    $.fn.svg = function () {
        return element = this;
    };

    $.fn.addClass = function (
        value) {
        if (!hasSvgNodes(this)) {
            jQueryFunctions.addClass.apply(this, arguments);
        } else {
            addClass(this, value);
        }

        return this;
    };

    $.fn.svg.test = function () {
        element.addClass("Red");
    };
}(jQuery));