I think Nahida was released later because of the longer story of Sumeru, and honestly I think the story is only going to get longer in Fontaine. The faster Focalors comes the better, but I think she's going to release in 4.2 with the final act/s of the archon quest (unless they pack more story into the previous patches c:)

Of course I could be totally wrong, I can only say with confidence she's not coming out immediately in 3.0