Recently DevExpress team took over the support of AjaxControlToolkit controls and from then on they have introduced nice and good features in AjaxControlToolkit. One such good feature they implemented is the removal of AjaxControlToolkit ToolkitScriptManager.
This change will resolves one of the common issues which most of the developers face when they use AjaxControlToolkit control. Usually as per design AjaxControlToolkit mandates the use of ToolKitScriptManager prior to using any controls. This will cause issues when user have to use asp:ScriptManager for any other controls.
Now with the latest release v15.1 You can use asp:ScriptManger with AjaxControlToolkit controls.