Army Boxes first showed up way back during 2nd Ed
40k (4th Ed Fantasy,
IIRC). Blood Angels, Orks, Orcs & Goblins and High Elves got the first treatment - basically a lot of units and a points value (and list) inside a box. Eventually other armies got them (Eldar and Tyranids I remember - I bought the Tyranid one). Really it was a box with a bunch of other boxes inside it. I remember the Blood Angel one saying that it was
"Over 2000 points!", which was true, as the included list came out to 2001 points.
During 3rd Ed
40K and some of 4th these shifted to limited release Army Boxes that came out whenever a new Codex came out. The Chaos Space Marine army box is actually what got me back into
40K after a hiatus, coming with a bunch of Marines, Rhinos, the new Oblits and Raptors that had just come out, plus the 3.5 Codex. They were a way of getting the new releases and the Codex right away, plus enough stuff to make an army, and usually a limited edition model of some sort (Possessed Marine, Imperial Guard standard bearer, Lictor, Necron Lord, etc.).
Then after a while they went away (the last one I got was the 4th Ed 'Chaos' Codex army box), and then we got 'Spearheads', which were essentially the same thing except that they were only the new models (not a complete army) and I don't think they had the Codex. These weren't around very long and they weren't very useful. The Marine one (5th Ed) came with all the new special characters, which was 99% Ultramarine and 1 Crimson Fist. Not very useful.
Eventually someone upstairs realised that we were getting a discount by buying these boxes, and so they vanished.
What's
more likely is that their initial stock wasn't selling as fast as they wanted to, leading to inventory sitting in stores for long periods of time wasting shelf-space - my local
GW, at the time, had a Dark Angel (4th Ed) army box that sat there for months, and a Tyranid (4th Ed) army box that sat there for so long (over a year) I was able to get it at a discount and even haggle the price!
Occasionally we'll see a 'Mega Force' show up around X-Mas, but that's about it.
And thus ends today's useless trivia lesson.