It is possible that this is because Labour Brexit policy at GE2017 was so close to Conservative policy. A small difference leads to a small importance.
However, are Labour party policy to change to, say, EEA + Customs Union, then the larger difference in policy might make it more important.
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However, are Labour party policy to change to, say, EEA + Customs Union, then the larger difference in policy might make it more important.