I believe this has to do with the fact that there are a number of controls that simply cannot be used with UpdatePanel control The following ASP.NET controls are not compatible with partial-page updates, and are therefore not supported inside an UpdatePanel control: * TreeView and Menu controls. * Web Parts controls. For more information, see ASP.NET Web Parts Controls. * FileUpload controls when they are used to upload files as part of an asynchronous postback. * GridView and DetailsView controls when their EnableSortingAndPagingCallbacks property is set to true. The default is false. * Login, PasswordRecovery, ChangePassword, and CreateUserWizard controls whose contents have not been converted to editable templates. * The Substitution control. * Validation controls, which includes the BaseCompareValidator, BaseValidator, CompareValidator, CustomValidator, RangeValidator, RegularExpressionValidator, RequiredFieldValidator, and ValidationSummary control. See this for details