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 such in several of the glossaries and we know Nemain isn't because she is at no point in the source material called the Morrigan or one of the Morrigan.
The complicated answer is that while Macha's connection to the Morrigan is easy to establish and fairly clear - she is repeatedly referred to as one of the three daughters of Ernmas with the Morrigan and Badb, is listed as one of the three Morrigans with the same two sisters, and acts along with them in stories - Nemain's connection is more convoluted. We can clearly say that she is a war and battle goddess and she acts in ways that are similar to what we see the Morrigan doing. She is also very closely tied to Badb, who is one of the Morrigan. In fact some scholars including RAS Macalister have theorized that Badb and Nemain originally formed a grouping together and only later did Badb become associated with the Morrigan, and then even later the Morrigan and Badb became connected to Macha.
Badb and Nemain share similar epithets including 'red' and 'red mouthed' and Nemain is sometimes referred to as 'Nemain, that is the Badb' or 'Nemain that is the Badb Catha'. It is likely that Badb's name like the Morrigan is also a title but in this case we can see the use of it applied to Nemain indicating their close ties to each other. Nemain is also referred to as Be Neit which may mean wife of Net* or woman of battle and is itself a name or title we see applied to other war goddesses. Nemain is said to be the wife of Net along with Badb in some sources, while others say she is his wife along with her sister Fea; unlike the three daughters of Ernmas Fea and Nemain are daughters of Elcmar. This is not a clear subject however, with some scholars like Heidja favoring the idea that Nemain is Badb's true name, while others like Gulermovich-Epstien seeing Nemain as one of the multitude of Morrigan goddesses but indirectly connected.
Pagans often tend to view Nemain as one of the three Morrigan because she is listed as such in modern books, possibly because a late 19th century book by Hennessey on the Morrigan discussed Nemain quite a bit. We can say with certainty that she is not one of the three if we are looking specifically at that specific grouping, but we can also say that she does appear together with Badb inciting battle and causing strife. While we can confidently include her among Irish war goddesses whether or not she is one of the Morrigan per se will probably always be an open question.
*Net is an obscure war god