Now it's mostly behind us, but there used to be a time where pypi didn't have wheels (a 2012 thing), or manylinux wheels (a 2016 thing) for most libraries. pip install was a world of pain if you didn't have the "correct source packages" in your system.
And now several of those projects built back then, they're no longer projects but deployed systems, might as well stick to what is working.
The fastest graph library I know, graph-tool, needs to be installed with Conda too.
https://pypi.org/project/graph-tool/
that's not a limitation of pip, that's a limitation of the maintainers of graph-tool
EDIT: But let's not pretend getting cibuildwheel going for every supported variant of PythonMajVer/OS/(32/64) bit is an easy lift for small open source projects. Conda is still more ergonomic than this, although it's slowly dying because big projects have the manpower to stand up and maintain the CI pipeline for PyPI, so Conda is less and less necessary.