Because by 2024 it'll be clear to everyone Brexit was a bad idea, so Labour, as a credible alternative government, needs to be able to say they offered people a way out.
They'll say "Labour's 2017 manifesto committed to Brexit. Corbyn couldn't make up his mind whether he wanted Brexit or not, and you *bailey/kinnock/nandy* opposed a 2nd referendum. So BOTH parties are responsible for our current difficulties."
The only way Labour can even begin to wash its hands of this mess is if those leading the party are the ones who said that once Brexit was negotiated, people should have a chance to accept or reject it.