Because "downgrade is not the right word, the right one is "optimization". It's possible that CDPR had to tone down few things here and there is order to achieve stable fps.
BUT
You guys here are exaggerating, it's nonsense to compare screenshots with different locations, time of day and camera angles and call it "obvious downgrade".
Fact is that we haven't seen gameplay on Ultra settings, who knows maybe screenshots too ?
Maybe CDPR is not showing it on purpose (this was kinda confirmed by Damien at GDC) so they don't upset console gamers when game comes out and it doesn't look so good as gameplay they shown, so kids won't start screaming "false advertising" and "give me refund" kind of thing.
That's the very definition of downgrade.
Yep, downgrading is pretty much toning down. Some people associate the word downgrade with something more serious like Dark Souls 2 or Watch_Dogs, but it can also be used if you look at games like Far Cry 3 which had a very minor visual downgrade.
Also, people have been complaining for awhile about the visual quality of The Witcher 3. I'd say the first time I saw someone mention a downgrade was about 6 months ago or so. The idea that not showing Ultra footage so console players wouldn't be upset is a little ridiculous. I think most console gamers are informed of the hardware differences in a gaming PC and a Xbox One or PS4 and understand that a PC offers higher quality visuals at the cost of a higher price.