What are two reasons why the author of the poem "Why So Pale and Wan" would use "do't" instead of "do it"?
One reason that a poet would use "do't" instead of "do it" is to make his meter work.
The two words "do it" are two syllables long, and may have stretched out the meter of the poem. The two words created an extra half a foot. By combining the words with an apostrophe, there are the right number of syllables for the poets chosen meter.