Because it's really really hard to justify. My dad is a chemist, and I recently learnt that some Fortran code he wrote in the nineties is till being used in ac

Because it's really really hard to justify. My dad is a chemist, and I recently learnt that some Fortran code he wrote in the nineties is till being used in academia. Try to explain to his colleagues that you need an on-site engineer to maintain python code just a decade old. Anyhow, while I disagree with the need of a dedicated engineer to port code to python3, porting isn't much of a big deal. If your dependencies work (eg: your libraries are python3-compatible), most of you work is done by 2to3. Very little effort is needed after that. The problem up to now, has been waiting for you dependencies/libraries to achieve python3-compatibility (recursively, of course). But we've already moved past that.