People hear socialist and probably quite often the thoughts that come to mind are -
State control, centralisation; corrupt one party councils; the worst versions of TU activity, bureaucrats and sectionalism; corrupt and authoritarian one party states, grim 1970s "municipal socialism", liberal middle class identity politics - 80s local government style; daft student impossibilism, headbanging cultist Trot paper sellers; run down public services staffed by demoralised jobsworths, Arthur Scargill, Derek Hatton and John Prescott thumping people

They think old, old, old fashioned.
Not saying this is right or even accurate versions of history in many cases, but -
This is a MAJOR image problem!
I say this as someone who would call themselves a green socialist or libertarian socialist (note lower case throughout!) But I know that when I stand as a Green I will get 10-15% and more (rising!) at local elections. If I stood as a "Green Socialist", though my politics would be no different I would be getting perhaps 3-4% max. Drop the Green from that and I would be getting the 1-2% normally achieved by left groups for the last 20 odd years!
The Respect experience mirrors this.
I agree with treelover and what L. German said - there is now a whole generation of people throughout Britain for whom socialism means nothing other than a lot of negative images. McAllion seemed an excellent candidate with gravitas and experience, in a byelection where people are more likely to break ranks with their usual party. Even accounting for a large tactical vote, you would have expected his share of the vote to rise, surely..