Dynamic controls getting lost after post back

if (!IsPostBack)
           {
               string tripid = "11850"; //Request.QueryString["tripid"].ToString();
               getStage();
               QuestionsPopulate("Booking");

           }

drpStage.DataBind();

protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            BindDataControl();
        }
        catch (Exception se)
        {
            //error if any
            Response.Write(se.Message);
        }
    }

    private void BindDataControl()
    {
        Repeater1.DataSource = GetData();
        Repeater1.DataBind();
    }

    //create dummy DataTable
    private DataTable GetData()
    {
        DataTable _dt = new DataTable();
        _dt.Columns.Add("Id", typeof(string));
        _dt.Columns.Add("Name", typeof(string));
        
        DataRow _drow = null;
        for (int i = 0; i < 5; i++)
        {
            _drow = _dt.NewRow();
            _drow[0] = i;
            _drow[1] = "Name" + i.ToString();
            _dt.Rows.Add(_drow);
        }
        _dt.AcceptChanges();

        return _dt;
    }
    
    public TextBox CreateDynamicControl(int id)
    {
        TextBox _txt = new TextBox();
        _txt.ID = "txt" + id.ToString();
        
        _txt.Height = new Unit(50);
        _txt.Width = new Unit(100);
        return _txt;
    }
    protected void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
    {
        if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
        {
                Panel pnl = e.Item.FindControl("pnlCointainer") as Panel;
                TextBox t = new TextBox();
                t.ID = e.Item.ItemIndex.ToString();
                pnl.Controls.Add(t);
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        foreach (RepeaterItem ti in Repeater1.Items)
        {
            try
            {
                Panel pnl = ti.FindControl("pnlCointainer") as Panel;
                TextBox ttx = pnl.FindControl(ti.ItemIndex.ToString()) as TextBox;
                Response.Write("TxtId = " + ttx.ID + "Txt Text = " + ttx.Text);
            }
            catch (Exception se)
            {
                //error if any
                Response.Write(se.Message);
            }
        }
    }