In sage 5.x the source code was quite small compared to today and could be patched with some effort. Today the project is huge and this would be impossible for one person to do.
It's vastly easier to just install sage using their own scripts from source and have some libraries duplicated. The sage source code build script is one of the simplest to use.
The problem is that sage actually depends on Octave, Maxima, R and many other packages. It parses binary output of many dependencies and often needs the exact version it expects because of that. It also has relatively brittle doctests testing everything an as a result often breaking on minor changes in dependencies.
If you have too much time, there is a very lengthy discussion on this on sage-packaging: https://groups.google.com/forum/#!topic/sage-packaging/ZJmJZ...
Something seems a bit weird about that to me, because when I used it ~8 years ago Sage was great and none of the features I care about changed.
Do can I get a meta-comment on why Sage might struggle to get packaged for Debian in the way that, eg, Octave/maxima/R/etc do?