I wouldn’t feel comfortable reducing an intricate poem like “The Waste Land” to a single main theme. It might be more considerate to discuss a few central themes. You can pick the one that you feel is the most important.
One main theme is desire. Within the first three lines, Eliot is already talking about desire. He seems to claim April is the “cruellest month” in part because it mixes “Memory and desire.” In the second, there’s more desire with the woman and her “strange synthetic perfumes.” Also, in part two, there’s desire with Albert, who wants a “good time.” Lastly, you could argue the water and rock imagery links to Eliot’s theme of desire. In the poem, you could say desire flows freely while, at the same time, it’s somewhat immovable or hard to act upon.
Another main theme is communication or lack thereof. There are several snippets of dialogue, yet it’s not always clear who’s speaking. Sometimes people are talking and there’s no quotes. For example:
I can’t help it, she said, pulling a long face,
It’s them pills I took, to bring it off, she said.
In these moments, it’s like the poem’s speaker merges with the characters in the poem to create one big character. You could argue all of the talking reveals the abundance of communication in Eliot’s world. Although, the ample communication doesn’t seem to lead to understanding. It seems to produce fragmentation, disquiet, and perhaps even miscommunication.
One more theme I can give you is that of tradition. You could argue the main theme is the history of literature. Throughout the poem, there’s numerous references and allusions to past poets and writers. For example, the rather famous “handful of dust” phrase comes from John Donne’s theological text Devotions. The “Unreal City” refrain comes from Charles Baudelaire’s poem “Les Sept vieillards.” There’s also numerous Dante and Shakespeare references. You could argue “The Waste Land” is built on the words and phrases of classic or renowned writers and texts. In that sense, you might claim tradition is the main theme. Without the tradition of literature, quite a bit of “The Waste Land” would be missing.