The second-last paragraph of ChrisF's answer is the rationale. :)
This site deals with the SE network in general so it's ill-fitting to have SO's rep carry over. Being an SO user with high rep doesn't make one qualified to comment on things for the English.SE site, for example.
At the same time, having high rep on English.SE very much means that the person is likely qualified to comment on the direction of English.SE, so their rep carries weight on English.SE meta.
In a way, you can think of StackOverflow meta as a separate StackExchange network site with its own rep.