Although it seems commonplace now, Wordsworth was adopting a radical stance for his time in focusing on humble and rustic life in his poems in Lyrical Ballads. Therefore, he feels he needs to provide some explanation. He does so as follows in his preface:
Humble and rustic life was generally chosen, because, in that condition, the essential passions of the heart find a better soil in which they can attain their maturity, are less under restraint, and speak a plainer and more emphatic language.
As the word lyrical (lyrical means emotional) in the title of the poetry volume suggests, Wordsworth's desire was to write poetry that evoked emotions, and he thought the rustic and common people showed their emotions more openly and in simpler language than the upper classes. He also goes on to state that rustic people live closer to nature and thus are more in touch with the passions that nature (which, to Wordsworth, is an expression of the divine force) can excite. He finds, as well, that the simple and heartfelt language of common people who do not have to hide their emotions is "more permanent" than the more convoluted language of the upper classes.
Wordsworth's autobiographical poem The Prelude is an excellent source for his thinking. In it, he writes that he was disillusioned after witnessing the bloody excesses of the French Revolution. However, he still believed the common people had wisdom and truths to communicate to the world and wanted to show such ordinary people in the best possible light. As he suggests in the preface to Lyrical Ballads, common rustics have important life experiences to convey and the kind of direct language that makes these experiences accessible.