That they're in coalition with the Conservatives, for one thing...
On a more general level, it's important to remember that TTL British culture hasn't had the pro-democratic shocks of the OTL World Wars. The view of the British ruling classes (of which the Social Democrats are of course a member too) is that the lower classes are kind of like farm animals: of course one should take care of them and be kind but one doesn't have to spend too much time taking their views into account. On the Social Democrats specifically, it's worth taking into account the three parties that make up the alliance: the Radicals are basically Fabian Socialists; Labour is the political wing of the unions; and the Co-operatives represent small-business owners and coops. In a more paternalistic political culture, the votes of the final 15-20% of the unenfranchised isn't considered particularly relevant to those three groups' political aims.