I'm not disputing that IronPython can be useful (although one also has to consider https://github.com/pythonnet/pythonnet, which wraps CPython into an IronPython-like projection). But if it can't share the library ecosystem, it might as well be a different language for all practical purposes. And it turned out that there aren't enough people who were interested in a Python/.NET combo for its own sake, so it remained niche. Any new implementation of Python on top of .NET VM would likely get similar reception for all the same reasons.