Markojp pretty much answered it above.
Because they finally get to a place where the snow is soft and relatively deep and the pitch is much lower.
On the steep part, remember the snow is hard. A fall means a slide to death most probably. The most important thing is to finish the turn with little energy that could cause you to keep going down and be out of balance cause regaining balance on that is risky.
It's difficult to find clips like that because it's not all that interesting to most people, and maybe more importantly it's difficult to film and show what it's like. Pov shots usually don't convey the steepness of the slope. That clip would've benefitted from pro cinematographers, but they'd have to make it there, then set up which would mean roping up. Let's not forget the route has been skied by 4 people in 30 years!
Check out this video of the same guys at Chamonix descending something with a similar pitch but with about 15" (40cm) of powder on it. There they do link turns since the powder acts as a brake.
Now check this descent in Peru out. Glen Plake, in blue, and Remy Lecluse make pretty much the same type of turn in very steep terrain with firm snow.
It should start at 3:09.
I'd recommend checking out the film Steep and Edge of Never.
Like most of this thread, this is complete BS. Wider skis don't take an intermediate, add soft snow, and instantly create an expert skier in real big mountain terrain. You still have to be able to ski it.
Rather, modern ski technology is an amplifier.
Of course you don't need a wider ski to ski powder. Any more than someone whose most narrow ski ski is ~98 needs a pair of GS race carvers to ski Sun Valley. But it is generally true that for most skiers, certain equipment is more optimized for specific environments and makes those environments more fun.
Modern equipment is an amplifier, not a crutch.
I'm don't think skidude meant the thread completely literally. I agree with what you say, everyone has been elevated, just like shape skis themselves have done. There's no way the astonishing performances in ski movies would be possible without wide skis. It's also true that they can be a crutch to some. Isn't it annoying when a snowboarder goes sideways in your steep powder? How about someone who basically wedge turns skiing your powder or just goes straight holding their ankles? You're right though, that's not what we'd call expert skiing, and you can't really begrudge someone for using technology to help them get out there. Still, the half life of powder is greatly reduced.
And also back in the day, the whole world had no colour in it. Everything was black and white, and different shades of grey.
What's that you say? Things had colour? No they didn't, If you don't have colour video of it or film that hasn't had colour added to it, it didn't happen.
You don't need skinny skis to ski powder, but you don't need skill to ski powder on fat skis.
On a related note, do you think a sport bike should have a one-way slipper clutch and abs, or will that ruin the rider's skill set?
But now we have 50 Shades of Grey!....
Skill is skill and will show up now matter what you're on.
Should we make Federer and Nadal play with the raquets Borg and McEnroe played with?
How about F1? Once they allowed paddle shifters it's different.