Lange had this to say about leaving the show:
“It ends up being a lot of time during the year being committed to something. I haven’t done that for a long time. It’s like doing a stage play between the rehearsal and the run. This is a 6-month commitment every year. That will be four years in a row. I want to have more time to myself I guess. Once it’s over and I have a full year ahead of me with nothing to do, who knows? It might not have been the best decision. But I think four years doing something is a sufficient amount of time.”
Now, this isn't very surprising. I think Lange has definitely run her course on AHS. In addition to this, there are some hints to the fourth season in terms of location and timeframe. From what we have heard, the show might be staying in New Orleans or moving Southwest to Santa Fe, NM. Another bit of info we got was that the fourth season will not be in contemporary times.