Because 8 is too many and 6 is clearly not enough. This is what separates the Steve Jobs' of this world from the rest of us.
Hitchhiker: Think about it. You walk into a video store, you see 8-Minute Abs sittin’ there, there’s 7-Minute Abs right beside it. Which one are you gonna pick, man?
Ted: I would go for the 7.
Hitchhiker: Bingo, man, bingo. 7-Minute Abs. And we guarantee just as good a workout as the 8-minute folk.
...
Ted: That’s right. That’s – that’s good. That’s good. Unless, of course, somebody comes up with 6-Minute Abs. Then you’re in trouble, huh?
[Hitchhiker convulses]
Hitchhiker: No! No, no, not 6! I said 7. Nobody’s comin’ up with 6. Who works out in 6 minutes? You won’t even get your heart goin, not even a mouse on a wheel.
Ted: That – good point.
Hitchhiker: 7’s the key number here. Think about it. 7-Elevens. 7 dwarves. 7, man, that’s the number. 7 little chipmunks twirlin’ on a branch, eatin’ lots of sunflowers on my uncle’s ranch. You know that old children’s tale from the sea.
So, I think that clears up why they went with 7 screens.
Why not 6? Or 8?
They said that they wanted to design a mobile Security Operations Center - do people doing this work normally need exactly 7 screens worth of information? I've seen pictures of "Operation Centers" which have tons of screens so it's plausible that someone would need "a lot" of screens, but I can't tell if this would actually achieve the goal of being a mobile SOC.
(My gut feeling is that they were trying to see how many screens they could put onto a single laptop, moreso than that small screen next to the touchpad actually being genuinely, uniquely useful, but I'd love to know for sure if each screen actually has a specific purpose)