It's a matter of prevailing ideological trends, certainly in Britain's case. In Britain since the beginning of the 20th century (at least) the prevailing trend was for more state intervention. Each new government intervened more than its predecessors in some way. The Liberal governments before the First World introduced old age pensions, insurance and unemployment assistance. After the First World War the Conservatives tried to remedy the depression in industrial areas ('special areas of economic assistance') by various interventionist means and extended some social welfare measures. They also introduced protectionist tariffs. During the Second World War the government directed Britain's war economy.

In economics there was in the interwar period a reaction away from the classical economic stress on non-intervention. Instead, new economic ideas by people like John Maynard Keynes advocated a central role for government in economic management. Also, people like R. H. Tawney, G. D. H. Cole and Harold Laski were influential intellectuals who were advocating socialism. Laissez faire was seen as industrial anarchy and chaos, a planned economy was advocated as utilising economic resources more efficiently through rational government direction.

So by 1945 there had been plenty of state intervention and nationalisation was not such a massive innovation. The intellectual trend was in favour of more planning and for the average voter the experience of mass unemployment in the 1920s and 30s meant that a planned economy with nationalisation and the promise of full employment was extremely attractive.

However by the 1970s there was a feeling from economically liberal people that nationalised industries were inefficient. The Institute of Economic Affairs had been founded in 1955 but it was not until the 70s that it really attracted notice and popularity. It advocated privatisation (or 'denationalisation' as it was then called). The reason was the economic crisis in the early 70s (the result of Arab oil crisis) that led to one million unemployed for the first time since WWII. For people like the IEA and Margaret Thatcher, government intervention had failed.

However this did not immediately mean that privatisation would follow. In 1979 the Conservatives only promised that they would run nationalised industries more efficiently than they had hitherto been. It was not until her second government (1983-1987) that privatisation took off in a big way.

The Labour Party opposed this but they lost four general elections in which they campaigned for more nationalisation. In 1994 Tony Blair was elected leader and he revised the Labour Party's constitutional clause IV which committed the Party to the "common ownership of the means of production, distribution and exchange". Under Blair's revision, no mention was made of common ownership. In effect nationalisation was deemed a vote loser and ditched.

However the recession that started in 2007 led to the government nationalising some banks and spending enormous amounts in a return to a Keynesian solution to economic crisis. So I don't think it's totally true to say nationalisation is now completely out of the question. Also, of course, some industries in effect no longer exist to be nationalised (e.g. coal and steel).

Recommended:

The Road to 1945: British Politics and the Second World War by Paul Addison. Thinking the Unthinkable: Think-Tanks and the Economic Counter-Revolution, 1931-83 by Richard Cockett