[Damien_Azreal]

They have not forgotten the series.

Gearbox is working on the next Duke game. But they've said after DNF this one needs to nail it. So they are taking their time, getting it set before announcing it.

They don't want to announce to early and fall into the same problem DNF fell into.
So right now they are in the planning and pre-development stages. Getting the story, setting... enemies... all that in place.
And most likely, they'll want to get a bit into the development before announcing anything.
Make a big announcement with the game, footage, most likely a release date.

It's only been four years.
People waited 14 years for DNF, from the day of it's announcement. Wouldn't you rather GBX announce something, and then we only have wait six or so months?

And, if you're thinking because E3 came and went with no announcement, that means nothing.
If you look in the past, Gearbox has never been "big" on E3. They prefer PAX Prime. They've said PAX is a more personal experience. They can interact directly with the fans, gamers and community.
Plus they always have their own panel at PAX.