Every proponent of a political ideology requires total policy adherence before they’ll accept their idealised version doesn’t work. They’ll no true Scotsman any and every implementation that could be termed an example of their ideology. For anything in a mixed economy they will claim the good things are caused by adherence to their policy ideals and the bad things by deviation and what’s needed is purity.
If by chance they do get in a position to achieve this they will scare the bejeezus out of voters because they will require a significant change and each change creates winners and losers and makes people fear they’ll be losers. So the more change the more opposition at worst and uncertainty at best. Also as you approach one persons idea of purity, by definition you move further from those who were previously your close allies and arguments with them intensify.
This then leads to defeat, which is then blamed on external malign forces (the anti growth coalition, the neoliberal right wing press) to ensure we can maintain the belief in ideological purity.
This includes various flavours of centrism before anyone says something. It’s a human trait not one particular to an ideology.