Back when Python 3 was initially concieved, the language just wasn’t that widely used, and mostly by enthusiasts. Some breakage wasn’t considered a big deal - it was expected users would easily update their code.

But during the time it took to design and deliver Python 3, the language exploded in popularity and reached a much wider audience and 3rd party libraries like numpy became crucial. So when Python 3 was ready it was a completely different ecosystem which was much harder to migrate, but I dont think the core team really relized that before it was too late.