Poetry does much more than Flit, like resolving dependencies, creating a lock file, and managing an environment where you can run your code. In particular, Poetry is meant to support application development (where you want to have a fixed version of your dependencies) as well as library development.

Flit is more aimed at being the simplest possible thing to put a package on PyPI, if that's all you want to do. It expects you to list dependencies in pyproject.toml manually.