Tennyson based his poem "The Lady of Shalott" on the Arthurian legend of Elaine of Astolat, who died of sorrow for her unrequited love of Lancelot. Tennyson uses the tragedy of her story in creating his poem, but he chooses not to focus on the 'why' or the 'how' of Elaine's curse, providing no backstory to the reader. Perhaps Tennyson feared that if he explained the curse in more detail in the poem, then the reader might not have paid as much attention to some of the other driving symbolism and action that later occurs.
Instead Tennyson utilizes the symbolism of the curse, having to weave beautiful tapestries and being isolated from the outside world, to speak to the Victorian ideal of how artists, or even poets, should remain separated from the rest of the common world. Ultimately, Elaine's choice to leave her craft and view the outside world with her own eyes spells her doom.