I'm aware of that, but Mobi was always proprietary, and logically, being oldest, or being in the majority, doesn't automatically make you better. Much as some of us, including myself, might wish that to be so.

Epub is the only format endorsed by the W3 (https://www.w3.org/AudioVideo/ebook/), the web standards body, and the only format that despite all its many imperfections, has any chance of widespread adoption. I'm not interested in rehashing the whole 'epub isn't REALLY a standard' debate, for all practical matter it is, and it's the only solid and truly free and open standard we're going to get in the foreseeable future unless everybody in the industry gets together and comes up with something cleaner. TODAY, it's the only thing out there even resembling a standard, for ebooks.

There are a lot of companies--Amazon among them--that have zero interest in preserving a free and open web, and would be delighted if they could kill it and control it. A significant part of that control is owning the data and the file formats.