Jquery code does not fires before postback

I am using jquery plugin for pop up calendar in asp.net page. When the page loads, it does not fires on focus event of corresponding textbox.

But, after postback the jquery calendar pop up works finely.

Please tell me the reason and solution for this.

Sample code:

<script src="Scripts/jquery-3.1.1.js" type="text/javascript"></script>
<script src="Scripts/jquery-ui.js" type="text/javascript"></script>
<link href="Scripts/jquery-ui.css" rel="stylesheet" type="text/css" />

<script type="text/javascript">

$(document).ready(function () {
$("#txtDob").focus(function () {
$("#txtDob").datepicker({
dateFormat: "dd/mm/yy", changeMonth: true, changeYear: true
});
});
});
</script>

<asp:TextBox ID="txtDob" runat="server"></asp:TextBox>