I wonder what the historical reasons are.
It was originally an internal tool, so I would guess either A) he doesn't have permission from all the contributors or B) he used reused code from elsewhere within Microsoft that wasn't open source compatible.
> I open-source pretty much all my work, but TextAnalysisTool.NET is an exception due to a variety of historical reasons. Sorry about that!
-- https://github.com/TextAnalysisTool/Releases/issues/22