From what I remember, it was because I needed an old version of the Amazon Kindle app, to download a less secured version of Kindle books, to then convert them to epub with an old deDRM plug-in. Essentially the whole process is not doable nowadays with stronger DRMs.
I don't know of any other book provider with an extensive catalog. Also Amazon often have authors into a exclusive agreement, so it's not like it will change anytime soon.
