There are some possible reasons listed here.
We could do with an official explanation from Labour HQ. So far all I've been able to interpret from various soundbites and putting 2+2 together is that:
- they are desperate to win
- they think the bill contains some items that the public would agree with
- they are scared their reasons for opposing would be misrepresented in the press as being anti-{insert items public agrees with}
- they don't back themselves to be able to counter this misrepresenation
- so they think opposing will harm their chances in 2020
Similar justifaction was given for abstaining from workfare vote in 2013, I'm not sure it went in their favour in the end, it certainly didn't make them win in 2015.
Andy Burnham apparently abstained because collective responsibility is important, why those that lead Labour thought abstaining was the best response is something that, for now, only they know. I repeat that an official explanation of their stance is required.