I can answer this one for Sweden.

The Swedish social democrats adopted several stances that made them very successful.

• They took a firm stand against violence and revolutionary change (to the extent that the party splintered over the issue) during the Spring 1917 food riots in Sweden (article by me on reddit).

• They took a firm anti-communist stance. Combined with the earlier stand on violence and revolution, this made them much better in the eyes of the old elite, who to a large extent accepted and worked with the social democrats rather against them. Compare for example the German social democrats, who cooperated with the communists and used socialist rhetoric, causing several large industrial companies to fund Hitler's nazi party.

• They absorbed some decidedly burgeouis values. Egnahemsrörelsen, a movement to allow regular people to build and own their own homes tied much more with the classical Swedish tradition and ideal of being a self-owning farmer than any socialist ideals.

• Keynesian economics during the Great Depression allowed the economic downturn to pass Sweden relatively lightly.

• The social democrats managed to completely subvert the sympathy from the Ådalen shootings in 1931 (5 dead, 1 of them a little girl among the bystanders) despite the demonstration and strike that caused it was the work of various local communist and anarchist organisations.

• The unions and the employers managed to reach an agreement in 1938 to resolve their differences between each other, without stat interference (this system exists to date, there is no minimum wage in law in Sweden, instead it is negotiated between unions and employers). The social democrats, supporting the unions and vice versa have held their part of this agreement, and the state has mostly stayed out of labour laws, making them even more palatable for the industrial elite.

• Forming a coalition government of all parties except the communists during ww2 showed a generosity and a willingness to put the wellbeing of the country before petty political gains and allowed the social democrats to gain massive prestige.

• After ww2, Sweden was in the enviable position of having a modern industry and large raw material production untouched by war and entered an unpredecented boom era, where the social democrats managed to husband the massive influx of money (despite comparably low taxes - Swedish taxes did not become internationally high until the 1970s) to build infastructure, new housing and extensive pension and welfare systems. Since the social democrats were in power during this era, they get a lot of the credit for the economic and social welfare growth during that time.

So summarise, they took a firm stand against violence and revolution, took a firm anti-communist stance, absorbed some very burgeouis values, subverted communist and anarchist sympaty, used Keynesian economical policies to steer Sweden past much of the Great Depression, put petty political gain aside forming a grand coalition government during ww2, presided over the superb boom years and built a well-functioning welfare state post-ww2 and still get a lot of credit for that.

I do research on left parties in Finland and know something on the subject. I would say the single major reason for the great influence of leftist parties in Finland, is Finland's defeat in WW2 to Russia.

The most important period was after the war, leading up to the early 1980's, with the most important legislation being done during the 1970's.

The backlash against political forces that had campaigned for war channeled support for the (previously banned) national democrats/communists and as an alternative, to the Social Democrats. Also the Agrarian League/Center Party was instrumental in most policy attributed to Social democrats. Their power came in part from the the long standing president Urho Kekkonen, who's political position was enforced by Finland's relationship with the Soviet Union. He also kept the political field fragmented and kept right wing parties out of government.

At the same time, labor unions were being strengthened by growing industrialization and a growing industrial work force and they became a political force that supported the Social Democratic party and the national democrats. Unions were in fact a very important element in all this.

This political atmosphere led to the Social Democrats and the Agrarian League/Center Party in cahoots with the left wing national democrats and communists being able to push through lots of political reforms which led to the creation of a welfare state and Social democracy.

Lots of pre-war stuff isn't being mentioned here, but I'd say losing to the Soviet Union was totally instrumental.

There's a working paper someone posted in my own thread yesterday "The Nordic Model: Conditions, Origins, Outcomes, Lessons" (2009): http://www.hertie-school.org/fileadmin/images/Downloads/working_papers/41.pdf.

The basic idea is that social democracy thrives in Scandinavia partly because of political culture (emphasis on consensual governance, social cohesion) and partly due to the way history unfolded. There is also an element of "path dependence": Scandinavia started with social insurance programs pretty early, and that set the course for what came later.