But no Python support as they promised. So sad.
One issue with embedding Python is that it's difficult to sandbox - to securely limit what the embedded runtime, and hence (potentially malicious) custom functions, can do: > [The Python developers'] standard answer to "How do I sandbox Python code?" has been "Use a subprocess and the OS provided process sandboxing facilities" for quite some time. [1] JavaScript, OTOH, is designed to support secure in-process sandbox…
Mostly, I think it comes down to Python being designed as a systems language rather than a scripting language. Integrating Python would seem to mean either a weak security model or a special (subset) version of Python. Neither is really going to meet the fat part of the Bell Curve...people who just want to get things done in Excel. Applying a 'browser abstraction' to Excel is probably better than applying an 'OS abstraction'. Anyway, JS has been a part of .NET and VS since JScript. Python, not so much.