Why are Boeing 747-400s not used as much as they used to be for trans-Atlantic air travel?

I notice that Boeing 747-400’s are being used less on certain trans-Atlantic, and some trans-Pacific routes…nowadays it seems to be more of a mix of twin-engine aircraft, MD-11s and some 747-400s on the transatlantic runs.. they even use twin-engine jets like the 777 on some long-haul trans-pacific routes…what are some of the reasons for this?
To Alibi…there’s plans for a new 747, the 747-8 which is going to be the newest in a long line of very popular Boeing jets.
I want to fly someday on both the 747-400 and the 777..I think the ‘Triple-Seven’ is quite an achievement for Boeing.

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Tags: Boeing Jets, Trans Atlantic, Trans Pacific