DropDownList Values getting cleared after PostBack in ASP.Net

In my web application i am setting the EnableViewState="false" for particuler page. In that page i am having two dropdowns,this dropdown binding the data in Page_Load event. So when i am clicking the button that two dropdown values are getting celared. see the below sample code.

<%@ Page Title="" Language="C#" MasterPageFile="~/MobileMaster.master" AutoEventWireup="true"
    CodeFile="ViewMapTag.aspx.cs" Inherits="ViewMapTag" EnableViewState="false"  %>

<asp:DropDownList ID="ddlHashTag" EnableViewState="true" 
                                        runat="server">
                                    </asp:DropDownList>




Binding the Data in Page_Load Event:-

 if (!IsPostBack)
            {
ddlHashTag.DataTextField = "MT_HASHTAG_NAME";
                        ddlHashTag.DataValueField = "MT_HASHTAG_NAME";
                        ddlHashTag.DataSource = dtHashTag;
                        ddlHashTag.DataBind();
}