A combination of several factors:
First, only five of the Ringwraiths were present; and their powers seem to have
increased in geometrical progession when more of them were present.
Second, the wraiths didn't expect any resistance: they were used to everybody
being petrified in their presence; it may have shocked them that somebody
wasn't. Third, after wounding Frodo with the Morgul-knife they thought
they had already won -- if they gave it a little time. So, why bother
fighting further?
As to the second question, there's a bit of overlap with a
passage in the next chapter on which I will have a question; so I'll wait till
then before saying
anything.
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All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king.
