I think siteNameValue is being overwritten with the last characteristic_option_value in the loop (which happens to be pune), causing only that value to be assigned to u_site_name.

(function executeRule(current, previous /*null when async*/) {

    var characteristicGR = new GlideRecord('sn_ind_tmt_orm_order_characteristic_value');
    characteristicGR.addQuery('sn_ind_tmt_orm_order_task', current.sys_id); // Filter by the current order task
    characteristicGR.addQuery('characteristic.name', 'sitename'); // Filter by the characteristic name
    characteristicGR.query();

    var siteNameValue = '';   // Initialize an empty string to store the concatenated values

    // Loop through all the characteristics and concatenate their values
    while (characteristicGR.next()) {
        var tempSiteNameValue = characteristicGR.characteristic_option_value; // Fetch the characteristic value
        if (tempSiteNameValue) {
            // concatenate with a comma (or any other delimiter) if it's not the first value
            if (siteNameValue != '') {
                siteNameValue += ','; // Add a comma separator
            }
            siteNameValue += tempSiteNameValue; // Append the value
        }
    }

    // If we have any valid site name values, update the u_site_name field
    if (siteNameValue) {
        current.u_site_name = siteNameValue;
    }

})(current, previous);