Either they have a much smaller user base to begin with or they're offering something really enticing in exchange for the pain. The problem isn't that migrating from python2 to python3 is super hard, it's that people don't see any immediate benefits for doing so. Had python3 come with something really enticing (like a significant performance increase or a better parallelism story), a lot more people would have not only accepted a lot more breakages, but also switched years ago.