The only thing I can think of is she actually wants to move on from politics, but she doesn't want to look like she is abandoning the Ontario Liberal Party in case she wants to step back into provincial politics in the future. So, she says she wants to become the mayor Toronto needs and that means she'll have to walk away from her MPP position. Maybe she would like to become mayor, however, she must know she has a slim chance at winning, so she wants to walk away from Queen's Park like she is fighting the good fight.
I see her landing a very well-paying job after the mayor's race so she can make good money and she can step back into politics down the road.