First of all, despite many plausible answers given by various readers, I
believe Tolkien did not write this particular battle in a plausible way.
Which is why it is a frequently asked question in the first place.
My favorite answer is that Aragorn is a truly extraordinary man, almost on the
scale of Glorfindel. He may not shine like Glorfindel, but probably no
other man could have attacked the Nazgul. Frodo is almost as
extraordinary in his ability to call upon Elbereth. The Nazgul were not
used to opposition of any kind, and therefore backed off and waited for the
morgul blade to take effect. They guarded the Last Bridge, and Glorfindel
drove them off. Then they guarded the ford.
I only found this plausible after reading "The Hunt for the Ring" in Unfinished
Tales and Letter 210. There Tolkien explains that the Nazgul rely almost
exclusively upon fear as a weapon, and would have been shocked at Aragorn's
ability to
attack.
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"‘I think he was a silly little man,' said Councillor Tompkins. ‘Worthless, in fact; no use to Society at all.'
"‘Oh, I don't know,' said Atkins, who was nobody of importance, just a schoolmaster. ‘I am not so sure: it depends on what you mean by use .'
"‘No practical or economic use,' said Tompkins. . . .
. . .
"‘It is proving very useful indeed,' said the Second Voice. ‘As a holiday, and a refreshment. It is splendid for convalescence; and not only for that, for many it is the best introduction to the Mountains. It works wonders in some cases. I am sending more and more there. They seldom have to come back.'"
