Why doesn't ajax control work when page is loaded for the first time ?

I have used ajax toolkit in web forms with updatepanel in it. I have used MaskEditExtender over a textbox. It doesn't work when the page is loaded for the first time but works if a postback occurs on page of any kind. This is the issue for every AjaxControl in page. Why ?

<asp:TextBox ID="txtBoxNICNo" runat="server" onkeypress="return isNumberKeyCNIC(this)" ClientIDMode="Static" placeholder="XXXXX-XXXXXXX-X" MaxLength="15" CssClass="form-control"></asp:TextBox><asp:maskededitextender id="MaskedEditExtenderCNIC" runat="server" targetcontrolid="txtBoxNICNo" mask="99999-9999999-9" messagevalidatortip="true"
onfocuscssclass="MaskedEditFocus" oninvalidcssclass="MaskedEditError"
masktype="Number" inputdirection="LeftToRight"errortooltipenabled="True"></asp:maskededitextender>