alt Hacker NewsLicensing concerns resulted in DjVu being originally preferred over PDF by archive.org and WMF projects like Wikipedia. With baseline PDF now being unencumbered and the widespread existence of FOSS readers, PDF is both the de jure and de facto standard across even those sites.
Also, PDF caught up on size with JBIG2, and tooling/support keeps getting worse.
(Not so fun fact: if you punch "filetype:djvu" into Google right now, you can easily page through what supposedly is every DjVu file on the Internet as far as Google knows, which is not many: "In order to show you the most relevant results, we have omitted some entries very similar to the 300 already displayed." I learned this the hard way when I began wondering why a bunch of DjVu fulltexts I hosted never seemed to show up in Google or Google Scholar...)