For most MPs, turning up in the Commons is a waste of time and very boring. Most don't get to speak and most, most of the time, vote the way their party tells them to vote. Add to that, the antisocial hours. They drink and eat because they are bored witless.
Then there's all the ridiculous ceremony and collapsing building with limited office space, parking, accomodation. Parliament really need to be pushed into the 21st C. A new building in, perhaps, a green field site somewhere in the Derby, Nottingham, Loughborough area. An airport nearby, good road and rail links.

I think many of the Labour hierarchy know many traditional, especially older, Labour voters supported leaved the EU and are scared of further frightening those voters away.
Unfortuntely, many of those voters have already moved on to RefUK.
And younger, more progressive voters now see the Greens as a realistic alternative to Labour.
As I presume many in Labour understand, their 174 seat majority in 2024 was supported by just 20% of the electorate. Labour just need to look at Wales to see what their future holds in the UK and England.