If you look at the cold hard facts of publishing you would know why Gail Simone is exiting "Batgirl"
Actual Sales in dollars was heading down for the Batgirl title.
DC wants to have a healthy collection of titles. In order to do that they need to insure that the titles live up to the company's expectations.
Looking at the Diamond figures for the last 3 years I found that the title went up when she took the title, but has started a downward trend.
Stories are important to the business, therefore, companies first blame the writer for a loss of sales.
That is why Simone is gone, plain and simple.
Go forth and Do well