The EU was not initially supported by the left because it was designed to facilitate free movement of people and capital, and thereby accelerated the relocation of industry to low-wage countries, leading to deindustrialisation and unemployment in countries like the UK. However, The EU did also promote environmental (e.g. clean beaches) and employment protections (e.g. maternity leave, sick pay), which were under such threat from a Tory government that most of the left, including Corbyn, came to support it as a kind of bastion of social democracy. Corbyn made more speeches in favour of Remain than any other Labour politician during the run-up to the referendum, and his '7.5 out of 10' proved useful on the doorstep in opening the debate with convinced Leavers. After Leave won, Corbyn's position was to respect the result, and to negotiate a soft Brexit. This, along with a mildly social-democratic manifesto (admittedly moderate positions like not running public services for shareholder profit are now viewed as the outer shores of crazed leftism by our billionaire-owned press), won 30 seats back from the Tories in 2017, the first time since 1997 that Labour had gained as opposed to losing seats.
But 2019 was different. Starmer as shadow minister for Brexit (with the support of conference and the shadow cabinet, mainly Remainers) imposed the second referendum policy, against Corbyn's judgement, and Leavers were furious that their vote was going to be annulled. Of the 60 seats lost on 2019, 58 were Brexit seats. And, of course, there was the demonisation of Corbyn, which has continued ever since. Starmer has suspended or disqualified almost 60 left-wing Jews. He has failed to support strikers, in spite of the fact that public opinion is generally with them. And he's kept up the anti-immigrant populist rhetoric that avoids addressing the structural inequalities that blight our country. He wraps himself in the Union flag (whereas Corbyn drew attention to the plight of homeless veterans and proposed pay rises for the armed forces) and demands unconditional support for NATO, suppressing any serious discussion about how to end this dreadful war. He is an extremist.