If you've ever followed the C++ committee discussions you'll see they put a lot of time and effort into considering legacy code when introducing language changes. For better or worse existing languages are on a trajectory set from their inception that can't always be easily redirected. New languages are free of this baggage and can wildly experiment.
I wish languages were more willing to release breaking versions, like a C++ v2 or such. That's not to say languages don't already have breaking changes between versions (Python comes to mind), but it would allow people to start fresh and clean up obsolete designs and libraries.
You know the cleaning up probably won’t happen. If some dependency doesn’t work anymore because Python introduced a breaking change, then you stick with the old Python version.