Because there are many unknown, probably worth of being picked by some volunteers.
I think it'd be worth creating some place (HN subsection?) where a list of dead or unmaintained, still worth of attention, projects could be kept so that potential maintainers could be made aware of their existence.
Just a simple list, all text, one line per project with a bunch of fields: YYYYMMDD formatted date of last update | Name (resolving to link to the project page) | Short description | clickable short list of say max 5 tags so that users can find similar ones just by refining the search to the desired tag(s).
It takes so much time and effort though. It would really be a labor of love to take up someones abandoned piece of work thats probably got deep structural issues if the author decided to pack up and leave already.
It's abandoned, though.