Organisations have a memory. The Labour Party hasn't been the party of Socialism for a long time. It's the party for centrists. Which shifts to the right.
I was talking to my partner about this the other day, and he put the issue in a nutshell.
"The Labour Party needs to die, as it's the default option in people's heads if they don't want to vote Conservative and are vaguely left. Perhaps if it did die, folks would consider other options like the Greens."