Because it emphasizes the distinction between different parts of a website which in turn makes it easier to focus on individual things.
Why are so many people against white space when it hasn't been overdone? Obviously "overdone" means different things to different people but in this case the information that matters is still pretty dense.
I took some quick measurements. The previous version leaves 2% of my browser window as whitespace containing no images or text. By comparison, the new version is 53% whitespace. That seems pretty extreme to me.
You must be referring to the constrained width of the page, you can change that in the preferences if you really prefer having the text cover your whole screen.
There's a reason we have a character limit per line when writing code - it's much easier to read and understand short chunks of vertical text. Personally, I find it far more comfortable reading text which doesn't go above 600-900px in width and this has been a well known UX/UI guideline for the last 10 years. Having your eyes go from the far left to the far right of your screen is also very straining on any >21" screen.
These kinds of objections are predicated on the idea that white space is inherently wasteful.
But the real question is, why is it good to strain the eyes darting them across distances that large. What is gained by filling in the white space just to fill it?
I find shorter lines difficult to read, they require so much more eye movement than longer lines. When a site will let me I prefer the whole length of my monitor as the line length. In my experience I can read on my monitor for 12+ hours a day without eye strain.
Yes, though it also contributes to that "busy" look if you try to cram as many things as possible on the screen. The content might have a logical separation but spatially multiple types of that content would still be close to your focal point which makes them a distraction.
We live in an age where attention is a valuable currency and most people would likely leave your site if they can't quickly find the thing they're looking for. The fewer things you have on your screen at a time, the easier it is for them to find what they're looking for. It doesn't apply everywhere but I think it applies to a good amount of cases which is why lots of white space has become the norm.
Plug: I made https://github.com/phil294/density-userstyle which is a global userstyle to remove unnecessary paddings on common websites. Will probably have to add Wikipedia soon.
