Social democracy is the extensive delivery of income, goods and services by the state.
It obviously requires high taxation levels. But it also presumes a high degree of efficacy in centralised delivery and direction. This is clearly much easier to do in dealing with a culturally homogeneous population. If people have a fairly similar range of assumptions about how to live, how to act, right and wrong, patterns of life and so on, clearly it is much easier to design rules and centrally deliver services without too much ill-will and suited to the patterns of community life than it is if one is dealing with highly varied outlooks and patterns of life. What's more, given the obvious potential for waste and idleness from government delivery and government income, it is much easier for social pressure to minimise shirking if people speak the same language and share a common outlook.