Because prior to being an MP she qualified, she will not give up that golden egg.
A Salary survey says she was earning on average £45,000 as a GP Practice manager, and although the policy doesn't make clear what it was years back, she certainly wouldn't qualify on her MP wages now,
PMSS 2014: Practice Manager Salary Comparison 2013 - 2014
A valid question is, given she's a University educated woman, had a £45k(+) salary and no welfare, medical or mitigating factors, how did she manage to secure a house over others possibly more needy?
It also has to be noted that Haringey Council are and have been Labour run since the early 1970's. She's probably been a Labour supporter all her adult life, as it happens her mother is a Labour Peer, and she became a member of the Labour NEC in 2014. Not that I'm suggesting she used any of the above to 'jump the queue' but I suspect that someone in the know probably possible couldn't comment.