[Randallw]

I think the explanation is that because of the Temporal Cold War, events in Enterprise follow a new timeline. Originally the Klingon's were not initially encountered because a Klingon was being chased by Suliban taking orders from a faction in the future, and he fled to Earth. ALso when they wiped out a colony on a planet, the guy from the future said that wasn't supposed to happen. They solved it, but think of the consequences of the descendants of those people killed not affecting the future.