Scientific software has strong dependencies on the specific versions of software used. The debian package needs to play nice with a couple of hundred packages w

Scientific software has strong dependencies on the specific versions of software used. The debian package needs to play nice with a couple of hundred packages which are not the ones sage is building against upstream. 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.