Although she wasn't particularly far left, she was passionate and vocal about the very things Mair hated. Given that she was Mair's MP, and his nature, it's pretty clear why he picked her out rather than some random person. Bear in mind he collected press clippings on her. From her Wikipedia page:
Cox was born in Batley, West Yorkshire, and studied Social and Political Sciences at Pembroke College, Cambridge. Working first as a political assistant, she then joined the international humanitarian charity Oxfam, where she rose to become head of policy and advocacy at Oxfam GB. She was selected to contest the Batley and Spen parliamentary seat after the previous incumbent decided not to stand in 2015. She held the seat for Labour with an increased majority. In her maiden speech she talked about the diversity of her constituency saying she had noticed that "we are far more united and have far more in common with each other than things that divide us". She became a campaigner on issues relating to the Syrian Civil War, and founded and chaired the all-party parliamentary group Friends of Syria; an obituary appearing in The Independent on the day of her death described her as having "campaigned tirelessly for refugees".