Why do some traditions *not* celebrate Christmas or Easter in any official fashion?

Following-up to these previous questions , why do some traditions *not* celebrate the birth and death (or, more accurately, His resurrection) of Christ as "official" church "events"? I understand that celebrating the Communion Table is a celebration of Jesus death - but why not celebrate His birth and resurrection, too, in a "special" way (ie beyond just celebrating His resurrection every Sunday by that being the the church comes together for worship)?