after working around the issue, the solution was :

 var jsonData = JSON.stringify(data);
     $.each(JSON.parse(jsonData), function (idx, obj) {
        $("#ddl_location").append('<option value="' + obj.id + '">' + obj.location + '</option>').selectpicker('refresh');
     });

step 1: stringify received object
step 2: use each function instead of for loop
step 3: parse the data using JSON.parse(jsonData )
step 4: refresh drop-down list every time you append data to drop-down list using .selectpicker('refresh')

remember to append $select.append(data).selectpicker('refresh'); when you don't get data populate in drop down

The code seems to work properly.

var jsonData = ["string 1", "string 2", "test 07 ", "here is test"];

for (var i = 0; i < jsonData.length; i++) {
  $("#ddl_location").append('<option value="' + jsonData[i] + '">' + jsonData[i] + '</option>');
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<select class="selectpicker" id="ddl_location">
  <option select="selected">Select Location </option>
</select>