Serious answer: it was judged that the act of scanning and delivering the book was ‘copying’ it - which copyright laws say they need permission to do. The whole ‘we only let one person at a time read it’ thing was deemed to be irrelevant.
Note that I’m not commenting on whether a reasonable person would also interpret copyright law this way, or on whether the laws are fair, just what the judgment was.
Because billion-dollar capitalist rent-seeking bullies got their panties in a wad and threatened to bankrupt them with legal fees that the bullies wouldn’t even feel if they lost. And the archive is just a non-profit that were offering a not-for-profit service for the betterment of society, with literally zero actual negative consequences.
Trying to normalize a DRM system is not advancing the betterment of society. If IA wanted to be pro-social scofflaws, they should have published DRM-free epubs/PDFs/etc for download, copyright law be damned. If they wanted to be law-abiding, they should have declined to offer this service, and informed users of the existence of shadow libraries.
Note that I’m not commenting on whether a reasonable person would also interpret copyright law this way, or on whether the laws are fair, just what the judgment was.