For me the two issues are: 1) Sage, which I use, is a huge project incorporating lots of other scientific software and using loads of Cython. It simply takes t

For me the two issues are: 1) Sage, which I use, is a huge project incorporating lots of other scientific software and using loads of Cython. It simply takes time to migrate all the dependencies and then Sage itself to Python 3, and it's not really a priority for anyone. 2) It's not really something that anybody seems to focus on a lot. Scientists mainly worry about science, minor differences in the programming language used are generally of little concern. Migrating to a different version is therefore not something most of them would spend time on, unless forced to by external circumstances.