Dave Filoni reminds Nerdist that the Empire is not a problem unique to the team of Star Wars Rebels; it’s a problem for the Star Wars universe as a whole.
It’s the type of character development and care that you typically don’t tie to television geared towards younger audiences, but then you remember it’s Disney and it all makes sense.
Sabine stays because she feels an obligation to Mandalore, her mother, and the actions of the Empire.
In terms of the story, it makes more sense for Sabine to stay with her family and fight for her homeland than to jet back off into the galaxy with the Ghost crew.