MtGox has to run the trading engine all on one server.
In related news the entire NASDAQ must run on a single server because it is impossible for trading engines to span multiple servers.
Well, not quite true. They split up ticker symbols across multiple servers by alphabetical ranges. But your point stands, that the NASDAQ engines probably each handle about 100-1000x more order flow as MtGox does. Whoever wrote Gox's software deserves to lose their CS degree, if they ever had one.