Via Ign.com:

An upcoming Star Wars novel will shed more light on why Leia abandoned politics for the Resistance military.

USA Today reports that Star Wars: Bloodlines will fill in some gaps between Return of the Jedi and Force Awakens. It establishes that Leia actually was a leader in the New Republic, and gained a seat on the Galactic Senate. However, during the course of the novel she starts to see a new generation of leaders that refuses to learn from the past of the Empire. One new character, Ransolm Casterfo, is described as Leia’s political opposite and even likes to collect old Imperial propaganda.

Star Wars: The Force Awakens – Why Leia Didn’t Become a Jedi
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Author Claudia Gray says the book is ultimately “about the role she’s created for herself since the fall of the Empire, and the one she takes up by the time of (The Force Awakens).”

The background of Leia’s role as General in the Resistance wasn’t made explicitly clear in The Force Awakens. Long-time fans of the Expanded Universe noted it was a pretty striking change from the character in the dozens of now non-canonical Star Wars novels, where she became a respected politician and even head of state.