"Good documentation" might be a bit of an overstatement. The second you get off the beaten path with a Raspberry Pi, things can get pretty hellish. Try bare metal C on just the second rendition, and weep before the non-existent documentation for the SoC they used. The official response to this issue (to paraphrase) was "it's fairly close to the previous version; you can figure it out." I don't want to know what sort of suffering you would have to go through trying to do bare-metal programming on an RPi 3.

There's at least a forum section dedicated to it, with a few posts containing collections of resources: https://www.raspberrypi.org/forums/viewforum.php?f=72

That doesn't change the fact that the documentation you need isn't there, and that they have no plans to provide it. Can you figure it out? Sure. Am I going to pull my hair out doing trial and error trying to figure out the GPIO pinout for an SoC that they have no interest in providing documentation for when I can just buy a product with better documentation? Of course not.

This is a new product from an established company. The original Beaglebone was released in 2008, 4 years before the original Raspberry Pi, and is far better supported from a technical perspective (meaning PCBs and spec sheets, and not umpteen unanswered forum posts about the same issue with the blind leading the blind).

BeagleBoard is fairly well established. The BeagleBone Black was one of the most popular boards by survey on hackerboards.com—even a couple years after the then-latest revision (2014)—second only to RasPi products, which were undercutting BBB on price.