The problem is that the entire ARM economy tends to have rather spotty support as a whole. There are mountains of proprietary peripherals baked into the chips, so you can't just pick and choose better supported options like you can on PCs with removable parts.
And while a chip manufacturer may make a handful of refreshes, most ARM platforms tend to only last a few years before being completely abandoned in favor of new ones. I am frankly amazed they can still get the BCM2835.
You can't forget BeOS, currently alive in the form of the open-source reimplementation Haiku. In spite of their perpetual Alpha status, I'm still continuously amazed that it's even half as good as it is.
You're missing a lot of the costs associated with publishing. You're completely omitting the costs of distrobution (which can be a fairly huge sum if you plan on having that book in stores) and marketing (which is going to be huge if you plan on having in stores, since you will want to sell a large volume of them).
It's not incredibly surprising to me (though I am surprised there was no forward announcement before the preview release). It became clear a while back that Microsoft was looking to expand it's developer environment to other platforms when they announced ports for .NET and SQL Server. Don't forget that Visual Studio Code has been cross-platform since inception.
The irony of this is that the only problem he really fixed was making his handwriting more regular. While it does help, the real problem is his malformed glyphs. His lowercase 'q' has such a large tail that it adds an extra space between the word, and his 'o' is written slightly different every time he writes it. The base of his problems seems to be that he writes with a mixture of different scripts, and if he taught himself to write with just one of them, he would be much better off.
And while a chip manufacturer may make a handful of refreshes, most ARM platforms tend to only last a few years before being completely abandoned in favor of new ones. I am frankly amazed they can still get the BCM2835.