To answer this question you need to understand Hardy's own philosophy of life and how he viewed events and tragedies such as the sinking of the Titanic, which is the concern of this poem. What is notable about this poem is that Hardy does not dwell on the loss of life. What concerns him is the way in which the sinking of the Titanic, the ship that was supposedly unsinkable and was the triumph of human achievement, is a demonstration of how the gods or fate chooses to attack humans that are arrogant in their own capabilities and powers. Note the reference to "human vanity" in the first stanza:
In a solitude of the sea
Deep from human vanity,
And the Pride of Life that planned here, stilly couches she.
There is an intense irony in the scene of the Titanic "couching" quietly, fathoms from the pride and hubris that led, Hardy argued, to its sinking.
This poem therefore establishes the futility of man's attempts and efforts to gain control over an indifferent universe that demonstrates how powerless mankind really is. The Titanic was the height of technological sophistication at its time, and thus its sinking confirms Hardy's philosophy of an "Immanent Will" that "stirs and urges everything" against mankind. Humans are shown to be ultimately powerless and defenceless against the might of nature and the divine, and this is powerfully expressed, according to Hardy, in the sinking of the unsinkable ship.