因为tag 查询变量期望值为术语slug。它将寻找slug“15”这个词（可能不存在）。

是的，这很令人沮丧wp_dropdown_categories() 使用ID作为值，而不是slug。这是因为它最初只用于类别（通常使用ID而不是SLUG），而不是一般的分类法。

然而，有this ticket on it. 我已经创建了this gist, 可以让你wp_dropdown_categories() 使用slug而不是id作为值。

供参考（插件中应包含以下类）：

/* 
 * A walker class to use that extends wp_dropdown_categories and allows it to use the term\'s slug as a value rather than ID.
 *
 * See http://core.trac.wordpress.org/ticket/13258
 *
 * Usage, as normal:
 * wp_dropdown_categories($args);
 *
 * But specify the custom walker class, and (optionally) a \'id\' or \'slug\' for the \'value\' parameter:
 * $args=array(\'walker\'=> new SH_Walker_TaxonomyDropdown(), \'value\'=>\'slug\', .... );
 * wp_dropdown_categories($args);
 * 
 * If the \'value\' parameter is not set it will use term ID for categories, and the term\'s slug for other taxonomies in the value attribute of the term\'s <option>.
*/

class SH_Walker_TaxonomyDropdown extends Walker_CategoryDropdown{

    function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ){
        $pad = str_repeat(\'&nbsp;\', $depth * 3);
        $cat_name = apply_filters(\'list_cats\', $category->name, $category);

        if( !isset($args[\'value\']) ){
            $args[\'value\'] = ( $category->taxonomy != \'category\' ? \'slug\' : \'id\' );
        }

        $value = ($args[\'value\']==\'slug\' ? $category->slug : $category->term_id );

        $output .= "\\t<option class=\\"level-$depth\\" value=\\"".$value."\\"";
        if ( $value === (string) $args[\'selected\'] ){   
            $output .= \' selected="selected"\';
        }
        $output .= \'>\';
        $output .= $pad.$cat_name;
        if ( $args[\'show_count\'] )
            $output .= \'&nbsp;&nbsp;(\'. $category->count .\')\';

        $output .= "</option>\\n";
        }

}

使用

$args=array(
    \'walker\'=> new SH_Walker_TaxonomyDropdown(), 
    \'value\'=>\'slug\', 
     .... 
);
wp_dropdown_categories($args);