They had reasons for choosing Oregon.
1) Not as polluted as, say, the cesspool that is New Jersey.
2) Lots of mana lines.
3) It had been cleared out thanks to the war, and other things.
4) No one was really paying attention to it, since Denver, Seattle, and other such places were keeping everyone's attention.
5) The site of the old Blood Wood later got renamed Cherynoble.