Why Is Macha Considered One of the Morrigan and Not Nemain?

For day four of my 30 Day Content Challenge I'm answering another question from social media - why Macha and not Nemain?

I'm interpreting this question to mean why is Macha considered one of the Morrigan and Nemain isn't, which is both a straightforward question and also a layered one.

The simple answer is that we know Macha is one of the three Morrigan because she is referred to explicitly as