Complex sentence: This is revealed to be because all references to the Beast in this dimension were magically erased, however Angelus was unaffected as he did not, technically, exist at the time when the spell was cast.
Split into simpler sentences: This is revealed to be because all references to the Beast in this dimension were magically erased. However, Angelus was unaffected as he did not, technically, exist at the time when the spell was cast.