Because publishing a package on PyPI is trivial compared to publishing on the Anaconda cloud. For the latter, you need to build your package for every platform and Python version you want to support. So sou need to setup some CI. This is just such an overkill for a pure-Python package. It makes a lot of sense for a project with a nontrivial C extension though, and I do build a conda package for one such project of mine. For everything else, I publish just on PyPI.