In TeX the line breaking depends on the whole paragraph. I imagine it could be visually quite distracting if the line-wrapping jumped around in a WYSIWYG word processor as you added text to a paragraph.
Edit: ah. I see this is noted in the posted article’s To-do section:
Figure out how to deal with dynamic paragraphs (i.e. paragraphs being edited) as their ratios will change during editing and thus visibly move around.
I don’t actually use a WYSIWYG editor any more, so I’ll leave my speculation there.
Which is: Interpret your instructions very literally, so you still need to go over your document carefully to fix problems like urls running off the edge of the page?
Smart-arse joke aside, I agree with you that LaTeX is generally great, but it's not perfect, and sometimes it is difficult to make it do what you want.
Because word processor implementors are lazy and incompetent? ;-)
I think that probably most people don't realize there is something better, and can't even really tell the difference between typical word processor output and nice TeX-style optimal line breaks. It seems to me most people don't care that much about typography.
A document presented with good typography might be easier to read for lengthy periods without losing concentration. There might be fewer distractions, like rivers of space running through the text, hard-to-read shapes due to poor kerning, or chunky CAPITALS and long numbers where small caps and old-style figures would not have disrupted the flow. There might be subtle visual cues to help the reader understand the material more quickly, like moving captions and headings closer to the subject material or spacing out bullet lists a little so their items are clearly separated.
But in a world where most people using word processors don't know what a stylesheet is and emphasis tends to consist of centring text, setting it in bold, capitals, and double-underlined, and then hitting enter a few times either side to space it out a bit, I think we can safely assume that most people either don't know about the subtleties of good typography or just don't care. The world might be a slightly better place if leading word processors all adopted better typographical conventions by default, and a few of us would appreciate the ability to produce better quality results, but I'm afraid it's never going to be a selling point for most people.