In "The Slave's Dream" by Henry Wadworth Longfellow, what is a reasonable explanation of why the rice lay ungathered?

To answer this question you need to look at the poem as a
whole, from start to finish. As you read the first stanza and then look at the last
stanza and the way that the poem describes that the slave whose dream we have just been
able to see has actually died from exhaustion and maltreatment. Note how this last
stanza presents the slave:

readability="18">

He did not feel the driver's
whip,

Nor the burning heat of
day;

For Death had illumined the Land of
Sleep,

And his lifeless body
lay

A worn-out fetter, that the
soul

Had broken and thrown
away!

The last stanza thus
reveals the reason why the rice lay ungathered, as indicated in the first stanza. The
slave, worn down by days and weeks and months of slave labour in the "burning heat of
day" and mistreatment, as indicated in the "driver's whip," has collapsed, exhausted,
onto the floor, leaving the rice ungathered. He has entered a state of unconsciousness
where he is able to "return" to his homeland, ironically just before dying and gaining
the liberty that he so keenly desired.