c# Get Set return null

  public partial class _Default : Page
    {
        private string strFile = HttpContext.Current.Server.MapPath("~/Output/");
        private string strUpload = HttpContext.Current.Server.MapPath("~/Data/");
        private string strOutput = "" + DateTime.Now.ToString("ddMMyyy_hhmmss") + ".xls";
        private ReadExcel readExcel = new ReadExcel();
        private ExcelData excelData;
        private string sFileName;

        protected void Page_Load(object sender, EventArgs e)
        {
            excelData = new ExcelData();
            sFileName = string.Empty;
        }

        protected void btnUpload_Click(object sender, EventArgs e)
        {
            if ((fileUploadDialog.PostedFile != null) && (fileUploadDialog.PostedFile.ContentLength > 0))
            {
                string fn = Path.GetFileName(fileUploadDialog.PostedFile.FileName);
                excelData.fileName = fn;
                sFileName = fn;
                string saveLocation = strUpload + "\\" + fn;
                try
                {
                    fileUploadDialog.PostedFile.SaveAs(saveLocation);
                   // Response.Write("");
                    lblStatus.Text = "Upload Status: The file has been uploaded. " + excelData.fileName ;

                }
                catch (Exception ex)
                {
                    Response.Write(ex);
                }
            }
            else
            {
                lblStatus.Text = "Please select a file to upload";
            }
        }

        protected void btnDisplay_Click(object sender, EventArgs e)
        {

            lblStatus.Text = "test" + sFileName  ;
//bothe sFileName and excelData.fileName will return null Why?
            //DataTable dt = readExcel.readCutomManualID(excelData.fileName);
        }
    }
}

namespace TGSTax.Models
{
    public class ExcelData
    {
       // public string fileName { get; set; }
        string _fileName;
        public string fileName
        {
            get
            {
                return this._fileName;
            }
            set
            {
                this._fileName = value;
            }
        }

        //public ExcelData()
        //{
        // public string fileName { get; set; }
        //}
    }
}