In your case, the only cell you have is selected by default.
Clicking on an already selected cell does not trigger the Table1_SelectionChanged event.
Even in your example with the 2 columns, if you click on the first cell nothing happens.
If you really need to display the content of this only cell in the TextBox you must do it with an extra code line during initialization of the table.
To show this you can add in your test code at the end of the App_Start routine.
table1.SelectCell("Column1",0)
nothing happens.
If you change it to
table1.SelectCell("Column2",0)
the Table1_SelectionChanged is triggerd
and the second cell content is transfered.
Best regards