It’s more complicated than that. Poetry’s predecessor Poet started at about the same time as PEP 518 (independently), but it jumped head-first to pyproject.toml

It’s more complicated than that. Poetry’s predecessor Poet started at about the same time as PEP 518 (independently), but it jumped head-first to pyproject.toml when the community started using that file. A lot of Python packaging is like that, the community is very organic and it’s rarely what caused what, but similar ideas popping up around the same time and eventually converge.