Is there any reason these never took off?

tl;dr I created a site for making EPUB files for reading

EPUB files are a way to store readable information. They're basically an archive of HTML files with limited styling and some standardization (e.g. Table of Contents). Any set of html files can be turned into an EPUB pretty easily.

More importantly, they're very convenient. Because of their simplicity, they're easy to create and parse. Many apps support it, such as Instapaper, Apple Books, and Calibre. You can even download ones for the terminal.

But they're not talked about all that often. For one, Amazon only added support for Kindle recently. When people talk about backing up a website, they usually recommend or, if you want a local copy, httrack.

But those formats are abysmal for reading. By contrast, EPUB excels at this. I can drag and drop the file onto my Kobo and bring it on the train. Or put it on my iPhone via Apple Books.

In particular, EPUB is handy because it puts everything into one file. Sometimes sites will delete or move articles around and my bookmarks will disappear. If an article is important enough to me, I try to drop it in an EPUB and move it to my Dropbox.

I could, in principle, make an HTML dump. Firefox has pretty good support for that but its just not as convenient as carrying around one file. EPUB has the side benefit divorces me from needing a browser to read it.

So why aren't they more popular?

In a vain effort to boost their popularity, I've created an epub-creator complete with a running site. You can drop any website in and it will use article-parser to extract the useful text from it. There are a few samples on the website. Feel free to check it out and let meknow what you think!