If I had to guess, it's because people are quite excited about the language and they're interested in giving it some visibility. There's always lots of new technologies, but very few gain wide adoption.

A page of examples of the Zen of Python got almost 200 upvotes. https://news.ycombinator.com/item?id=2203101 and https://news.ycombinator.com/item?id=3697378

I enjoy the pragmatic reasoning behind the documented choices. It's an interesting input even if I don't work in Go.

It's more of a "best practices" than it is documentation. In the vein of Effective C++ by Scott Meyers. It's well-written, informative and interesting. Definitely one of the best resources on Go I've come across.

There is a collection programming books titled "Effective X", for language X. From multiple publishers, IIRC, though I think it may have started with Addison Wesley.

The majority of those books with which I'm familiar have pretty good if not excellent reputations.

I would suggest that this documentation's title may be acknowledging this, and perhaps -- deliberately or not -- leveraging it.

EDIT: No, maybe it was indeed O'Reilly. E.g. Effective C and Effective C++.