Because Labour could, potentially, win if the previously Labour but now non-voters can be persuaded to start turning out and voting Labour again, but only if the non-voting support base is fairly large, and
without that sort of analysis over larger areas it's hard to judge — it's definitely partially true in my ward, Labour had disappeared electorally years ago, but won the
seat in 2012, despite most other parties getting similar votes to
normal (and our support mostly going to an Independent we were tacitly backing).