Why 435?

There's probably nothing specific about 435px, but there's a good UX reason why the text doesn't take up the whole page:

https://wiki.c2.com/?TenWordLine

https://baymard.com/blog/line-length-readability

If you look at PG's blog, 435px comes out to just about 10 words, so he's hewing to the guidelines. A bunch of other sites do this as well: take a look at the main column of the Facebook News Feed, or the NYTimes. Google Search Results go a bit wider (about 600px = 12-13 words IIRC), but the general principle holds.