Why is Brexit a serious problem for Labour?

Why is Brexit a challenge for Labour?

I just had a look at the Labour Party target list for 2020, and it makes bleak reading. Here are the 20 most marginal Conservative-Labour seats and the corresponding EU referendum result in those seats (Source: Dr Chris Hanretty constituency estimates which are widely regarded as very accurate).

Gower: LEAVE: 50.2%
 Derby North: LEAVE 54%
 Croydon Central: REMAIN 51%
 Vale of Clwyd: LEAVE 56%
 Bury North: LEAVE 54%
 Morley and Outwood: LEAVE 60%
 Thurrock: LEAVE 70%
Plymouth Sutton and Devenport: LEAVE 51%
Brighton Kemptown: REMAIN 57%
 Bolton West: LEAVE 56%
 Weaver Vale: LEAVE 51%
 Telford: LEAVE 67%
Bedford: LEAVE 53%
Plymouth Moor View: LEAVE 69%
 Lincoln: LEAVE 57%
 Peterborough: LEAVE 63%
Cardiff North: REMAIN 61%
 Corby: LEAVE 60%
 Waveney: LEAVE 63%
Warrington South: LEAVE 51%

 Even more worrying when you consider Labour is losing many people who voted Leave to the Tories is when you look at the 20 Labour seats most likely to fall to the Tories at the next election:

City of Chester: REMAIN 53%
Ealing Central and Acton: REMAIN 72%
Brentford and Isleworth: REMAIN 60%
Halifax: LEAVE 60%
 Wirral West: REMAIN 57%
Ilford North:LEAVE 51%
Newcastle Under-Lyme: LEAVE 62%
Barrow and Furness: LEAVE 57%
 Wolverhampton South West: LEAVE 54%
Hampstead and Kilburn: REMAIN 77%
Enfield North: REMAIN 52%
 Hove: REMAIN 67%
 Dewsbury: LEAVE 57%
 Lancaster and Fleetwood: LEAVE 52%
 Derbyshire North East: LEAVE 52%
 Harrow West: REMAIN 59%
 Bridgend: LEAVE 50%
Middlesborough South and Cleveland East: LEAVE 65%
 Westminster North: REMAIN 58%
Walsall North: LEAVE 72%

This group is a mixed bag, however it is clear that a sizeable number of Labour voters in these Labour held seats did vote to Leave.

Now look at the 10 most marginal Labour-Liberal Democrat seats to look at their unique threat:

Cambridge: REMAIN 74%
 Burnley: LEAVE 67%
 Berdmonsey and Old Southwark: REMAIN 73%
 Cardiff Central: REMAIN 70%
 Birmingham Yardley: LEAVE 61%
Bristol West: REMAIN 80%
 Bradford East: LEAVE 58%
 Redcar: LEAVE 68%
 Norwich South: REMAIN 60%
 Ashfield: LEAVE 71%

 Again a mixed bag, but the Lib Dems will have a chance in at least a few of these seats.

 Now the UKIP threat: 10 Most marginal Lab-UKIP seats:
 Hartlepool: LEAVE 70%
 Heywood and Middleton: LEAVE 62%
 Dagenham and Rainham: LEAVE 70%
 Mansfield: LEAVE 71%
 Great Grimsby: LEAVE 70%
 Stoke on Trent North: LEAVE 71%
 Rother Valley: LEAVE 70%
Stoke on Trent Central: LEAVE 65%
Dudley North: LEAVE 69%
 Peninstone and Stockbridge: LEAVE 61%

 Simply put, there are no easy answers...but it does not look promising.