Because:
Morgan Le Fey (the original) is dead
She, most definitely, is dead. Of course, now that she holds
The Word of Kemmerler, she can probably improve her color and smell a bit. Mavra is Morgana le Fay *gasp*.
It seems to me that she wanted to "outlive" the original Merlin for some reason. So she permitted herself to be taken by a Black Court Master. Now, she intends to use the premier "Journal of Necromantic Medicine" to return to her live self. In the interim, she has been spending her time tearing down all that Merlin might hold dear:
1. She attempted to secure Excalibur by trading with her own Athame. Thwarted by Harry.
2. She used the Red Court as a cat's paw to start a war with Merlin's White Council (hoping to destroy it). Again, thwarted by Harry.
3. She attempted to use Outsiders/ sorcerers/ and thralls to destroy Demonreach, because it is one of Merlin's most exquisite accomplishments. Once again, thwarted by Harry.
In Blood Rites, Mavra shows up and stupidly attacks Harry, Thomas and Mouse for no reason with no result. The purpose was to let Harry know she's in town, and a challenge to hunt her down. She doesn't come out and play herself, because the whole basis of her visiting Chicago was to lead him by the nose to her lair. Assume he would bring allies, and compromise those allies with photos. Murphy worked out perfectly for leverage to secure Kemmerler's Word. She needs it to...well...come back to life.
Hell hath no fury like a woman scorned. Of course, Merlin anticipated this and is sleeping deep down in Demonreach well waiting. Not one to go in his sleep, indeed. Can we read your journals now, Ebenezer?
Truly, I think you can find direct and indirect connections to Mavra/ Morgana le Fay in every single book in the series so far.