Why does T.S. Eliot refer to lilacs in The Waste Land?

Eliot refers to lilacs at the very outset of his landmark
modernist poem, "The Waste Land":

April is the cruellest
month, breeding
Lilacs out of the dead land, mixing
Memory and
desire, stirring
Dull roots with spring
rain.

Eliot employs the conventional image of lilacs
connoting sensuality or romance  in other poems - earlier in "Portrait of a Lady"
(published in Prufrock and Other Observations, 1917) and later,
in "Ash Wednesday," (1930) - but in "The Wasteland" (1922), the image of lilacs he
presents in a manner striking in its newness. For this reason, the image of lilacs -
appearing as it does at the head of the poem (line 2) - acts as a kind of literary
Rosetta stone to the complexities of the poem. The poet accomplishes this by his
conscious allusion to the Walt Whitman poem,  "When Lilacs Last in the Dooryard
Bloom'd":

When lilacs last in the dooryard bloom'd,

And the great star early droop'd in the western sky in the night,

I mourn'd, and yet shall mourn with ever-returning spring.

Ever-returning spring, trinity sure to me you bring,
Lilac
blooming perennial and drooping star in the west,
And thought of him I
love.

Whitman's poem is a passionate elegy on the death of
U.S. President Abraham Lincoln, assassinated in the spring of 1865 when the lilacs were
blooming. In the poem's semiotic innovation lilacs - traditionally a symbol of the
renewal of the earth in spring - are now connected with mourning, and anguish and
death.  In "The Waste Land", Eliot takes the semiotic innovation Whitman supplies, but
then uses it, in the manner of a literary Rosetta stone, as a means to understanding the
poem's complex allusions to the modern culture of death.