Pip cannot find a specific version of VTK package although it is in PyPI

Question

I accidentally installed VTK 9.0.0, but want to downgrade to 8.1.2.

I uninstalled VTK 9.0.0 using pip uninstall but now I cannot install 8.1.2 for a weird unknown reason:

$ pip install "vtk==8.1"
ERROR: Could not find a version that satisfies the requirement vtk==8.1 (from versions: 9.0.0)
ERROR: No matching distribution found for vtk==8.1

The version 8.1 is definitely in PyPI. Why pip complaints and does not the needed operation?

Answer 1

This answer is based on @metatoaster's comment.

In this case, pip could not find VTK 8.1 because the package VTK is not available for Python 3.8 for versions < 9.0, while I used Python 3.8.