In "The Lady of Shalott," no information is revealed concerning who cursed the Lady, why she is cursed, or how long she's been cursed. The "history" of the curse is left ambiguous.
This means, of course, that those details have nothing to do with what the writer is revealing in his work. Whatever is on Tennyson's mind, "who" curses her is not necessary information, so as readers, we shouldn't spend much time on it. It's not a part of the work of art. It's irrelevant to what the work of art accomplishes.
What's signigicant is that the Lady seems to be completely contented and happy fulfilling her role as a separated artist, until the song of Lancelot draws her to the casement to hear and see for herself. Whatever else is going on in the poem, a rational explanation of and history of the curse is not.