It *was the most common letters, in order, once. It was also the order of letters on a linotype machine, the device that they used to use to set type for newspapers. The story is that if the type setter discovered some gross error that the proofreader had missed, he'd run his finger across that row and etaionshrdlu would appear in the copy. He'd send it back and it would get fixed. Only sometimes it wouldn't get fixed and the newspaper would go out onto the street with a line like:

President Hoover today announced that he etaoinshrdlu…