The thing to consider is that the NA servers are not purely just for NA players, there are other places in the world that actually connect to these centres and that's what SE (and Devs for many other MMOs) have to plan around. Players from parts of Central/Southern America, Oceania and even further afield may all connect to this one so the location has to suit more than just a single region, and California was the best suited location.

I know some players are adamant that it's an SE issue, but personally I don't think it is and is all down to the respective ISPs. Having played from EU on the Aether NA centre (on an alt before and after the move to Cali), I barely had that many issues at all regarding routing. Sure the occasional lag happened which was an ISP issue, but there was never anything that prevented me from playing the game at any time of day. So from what I found that being several thousand more miles away I still had less problems than those in the actual NA region, so who knows perhaps just like Saito just mentioned previously, US infrastructure could be to blame more than anything.