One of the themes of "The Lady of Shalott" is directly tied to symbolism present in the poem.
At its center, the poem is probably about the role of artists and the creative process. The Lady as artist is totally isolated from reality. Her view of the world is limited to what she sees in the mirror. She does not experience reality, but only a two-dimensional reflection of it. She is, symbolically, an artist who is out of touch with reality. She is not allowed to experience reality for herself, but only to view it through her mirror.
When she is drawn to reality (by Lancelot's singing, by the way--another art form) and looks out the window her art "explodes," for want of a better word. Her art crashes in the face of reality. The mirror cracks, symbolizing the failure of imagination alone to accurately reflect reality. The reality of Camelot cannot match her art portraying Camelot.
The idea seems to be that an artist cannot accurately portray reality if too drastically isolated from it. Again, the poem is concerned with artists and the creative process. Perhaps that's why, from the writer's point of view, the Lady of Shalott cannot be allowed to reach Camelot--Camelot, as she portrayed it, doesn't exist.