@Martin Originally, the plan was to have separate chat servers for all sites in the network. But that didn't pan out because those individual chat servers had very little traffic each, unlike SO's server which had tons of traffic by comparison. So it was decided to merge all the non-SO servers into a single one. That combined server had the same amount of overall traffic as SO's one.

Meta.SE has its own server for two reasons: first, because this server houses some SE-internal chat rooms which contain content that need to be separated from normal moderators (who have mod access on chat.SE but not here), and second, so the team can test changes to chat here on this low-traffic server before rolling them out to the much larger chat.SO and chat.SE servers.