I can only speculate. :-) Maybe restricting the width to improve readability would yield suboptimal results in heavily nested threads. One might find out by manipulating the corresponding css attribute. Doesn't chrome or firefox support this out of the box?

So this is the width of the paragraphs. I think it would still be useful if you could disable the automatic line breaking to provide your own formatting. I think there is a middle ground between the two regarding readability.

EDIT: Of course you can disable the automatic line breaking as I've done exactly this by indenting the whole post with 4 spaces, except for the links. But that also changes the font to the monospace family.