My impression from the countries where I know the modern political history fairly well is that during the post war era being in power and being able to deliver goodies to the constituents was a good way to be re-elected. Events could - and did - disturb things and threw governments out.

The change comes with what we can call the neoliberal era, starting in late 70ies to early 90ies. Here being in power means taking the blame.

I think the difference is explained mainly by the difference in economics. In the post-war era full employment was a prime directive, which meant those in power had the task of directing what to employ people with doing next. This gave power to fulfill promises, as well as kept a decent proportion of the population reasonably satisfied.

In the neoliberal era low inflation in consumer goods and wages (but not in assets, for examples houses) is the prime directive, and it is upheld by keeping unemployment high enough. This gives the politicians in government much less power as much of the labour force needs to be idle.

If you can't anyway deliver it is probably better to be in opposition.