WHY TO USE NAMESPACE WITH _DEFAULT TO ACCESS WEBSERVER CONTROL IN CODEBEHIND

I have one simple form default.aspx, default.aspx.cs. with pane and textbox control. I have created simple default page with following declaration :
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
I have used ajax control -roundedcornerextenor to panel. and it works fine.
But in codebehind, If tried to use textbox1.text ='x'..then i couldn't get  textbox object at all.
 
Then after some research i change page directive to
 
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="
AJAX_RoundedCorner_Demo._Default" %>
then it worked. AJAX_RoundedCorner_Demo was my namespace.
Then i got access in default.aspx.cs file as
textbox1.text
But why I have to use namespace in page directive, Why It didn' ...