The servers are entirely different data centers on opposite sides of the world. It's not so simple as diverting resources. The JP servers are running smoothly because they've been stress-tested since early December 2012. The NA/EU servers have been operational for a combined... three weeks at this point?

Edit: Additionally, here's a post on reddit by server technician about a probably reason why we're having difficulties, and why it's not so easy to fix as "add more servers".