They point to the reason in the intro but don't make it explicit: it's because this is at the intersection of computing with a much older tradition, typesetting.
There's no "correct" way to typeset a document, there wouldn't even be a consensus among typesetters on what the implementation specifics look like. Rather than turn the spec committee into a decades-long ecumenical council of typographers they just left the specifics up to each individual "shop" as it always has been. Except now instead of printers it's the browser vendors needing to make the final call.
>There's no "correct" way to typeset a document
They can add multiple typesetting properties and allow the develop to decide which one to use. Besides, letting each browser decide what the "best" line break looks like doesn't solve the problem of there not being a definitive answer to that question. Even here, I don't think the Chrome developers have a vastly different opinion on what a good line break looks like. It's possible they didn't like the performance implications of webkit's version or had some other tangential reason, although the blog says performance is not an issue.
> Rather than turn the spec committee into a decades-long ecumenical council of typographers...
Having worked with passionate (aka opinionated) typographers, that phrasing earned a well-deserved chuckle. Leaving implementation choices up to each browser was certainly the only to way to get it into CSS. Hopefully various implementations will evolve over time and coalesce into a fairly consistent baseline.