If you you mean why isn't she going to be in the next JLA line-up? It's probably because her leg was broken in four places by Prometheus.
James Robinson had this to say about Vixen: "With regards to Vixen, you’ll see that she has a definite plan and goal in #41. It’s revealed that she leaves for a specific purpose. And that purpose will fold back into the Justice League and into the DC Universe in the future."