Pipewire was created to support containerized applications (with limited system access), to also support video, and to unify both pro low-latency audio (JACK) and regular low-power audio (PulseAudio). It might have been possible to retrofit this all into the PulseAudio project - but the authors decided to start from scratch. There are some interviews and talks on YouTube, they might have insights into why exactly that was the case. But I have not seen any indications that it was because PulseAudio did not work for what it was designed for.