My guess is that it's simply a tech challenge. Justifying text seems non-trivial, especially if hyphenation is used (which I believe is typically the case in print, otherwise you get huge spaces).
According to caniuse `text-align: justify` is still not widely supported [1]. At this point, with how long people have had to get used to non-justified text online, it probably wouldn't be commonly used even after it becomes widely supported.
The specs for the justify property illustrate some of the challenges in implementation [2].
[1] https://caniuse.com/css-text-justify
[2] https://drafts.csswg.org/css-text-3/#text-justify-property
My understanding is that justified text is harder to read as in order to create the justification, uneven spaces are generated between the words which disrupts the flow of your eye.
Additionally, just like it's harder for the eye to make out a word in all caps due to greater regularity in the image, it's also likely harder to distinguish differences in a paragraph if it's all a 'block' to the reader.
I'd imagine some people rely more heavily on the shapes of words rather than the letters or spelling itself. And then this heuristic might also carry through into the shape of a paragraph.
It’s because the primitive line-breaking algorithm used by browers, combined with the lack of hypenation in most cases, leads to a poor result, especially in narrow columns. Look up “greedy line breaking”.
Most people really do prefer justified text. Lots of webdevs probably get excited when they realize browsers have built-in justification too.
It’s not the default, which may play a big part. In addition, justification is an awful way to align text[1], making the composition uglier and harder to read, even if you don’t consciously notice it.
There are ways to work around those issues (all of them hacks: deforming characters and spaces in between), but even most professional design software doesn’t support the full range of features. Last I checked, Adobe Illustrator didn’t support that kind of granularity (though Adobe Indesign did) and neither did web browsers.
[1]: https://en.wikipedia.org/wiki/Typographic_alignment#Problems...