Why? Because they'd have more remainers on side now? I very much doubt that. Or more leavers? Even more doubtful: one leave constituency (well off vindictive pensioners in the south east) will never be talked out of Brexit, let alone into support of Labour. And it would have been hard to make a really compelling left wing case for Remain to the other leave constituency (disenfranchised working class voters in the north), because they'd heard all about how great workers' rights were under the EU, while experiencing something quite different. The only compelling left wing case for the EU they could make was negative: you will be even worse off if these bastards slip the harness of these other bastards. I think that's basically what they did. And everyone had heard enough of that kind of talk by then: no-one believed things could get better, or even much worse for them - they wanted to force a confrontation, and spread the pain around a little.
But why do I keep rising to the bait? This is just another variant of "If Liz Kendall were in charge Labour would be 20 point ahead in the polls!"