When I was a child computers didn't exist. We wrote by using mechanic typewriters. The hammer of the typewriters marked the accent mark at the same height of the top of capital letters. Then, capital letters couldn't take any accent mark, and it became in a kind of agreement. Whenever a word had been writen using capital letters, it was right that it did not take an accent mark.

However this kind of implicit agreement became in a gramatical rule, even some teachers taught that capital letters didn't take any accent mark. It was a great confussion that persists till nowadays.

Windows and graphics computer enviromnets overcame that limitation, however till now, you can read on the internet or  on TV screen, words writen without accent mark.