Because YOU CAN’T DEREGISTER JQUERY IN THE ADMIN

Why would you even want to?

See src: http://core.trac.wordpress.org/browser/tags/3.6/wp-includes/functions.wp-scripts.php?order=name#L105

// Do not allow accidental or negligent deregistering of critical scripts in the admin. Show minimal remorse if the correct hook is used.
if ( is_admin() && 'admin_enqueue_scripts' !== current_filter() ) {
    $no = array(
        'jquery', 'jquery-core', 'jquery-migrate', 'jquery-ui-core', 'jquery-ui-accordion',
        'jquery-ui-autocomplete', 'jquery-ui-button', 'jquery-ui-datepicker', 'jquery-ui-dialog',
        'jquery-ui-draggable', 'jquery-ui-droppable', 'jquery-ui-menu', 'jquery-ui-mouse',
        'jquery-ui-position', 'jquery-ui-progressbar', 'jquery-ui-resizable', 'jquery-ui-selectable',
        'jquery-ui-slider', 'jquery-ui-sortable', 'jquery-ui-spinner', 'jquery-ui-tabs',
        'jquery-ui-tooltip', 'jquery-ui-widget',
        'underscore', 'backbone',
    );

    if ( in_array( $handle, $no ) ) {
        $message = sprintf( __( 'Do not deregister the %1$s script in the administration area. To target the frontend theme, use the %2$s hook.' ),
            "<code>$handle</code>", '<code>wp_enqueue_scripts</code>' );
        _doing_it_wrong( __FUNCTION__, $message, '3.6' );
        return;
    }
}