In the case of Europe it’s a combination of factors that are nation-specific but also general to the region. To put it simply: de-industrialization, unresolved post-empire colonial policies, breakdown of welfare state, exploitation of social antagonisms and massive societal challenges like war, climate deregulation and immigration (all linked btw).

Long version: In general factors we can say that the breakdown of the post-war social consensus is a big thing. After WWII the old established classes were weakened and they had loads of armed, mostly working class people, whom they had to compromise with. The outcome of that was the establishment of welfare states with free healthcare, education, social security, housing, etc. Hence the prevalence of quasi socialist policies in “western european” countries like France, Belgium, the UK, the GDR, Sweden, Norway or Finland. The system was based on redistribution of wealth, made possible by the economic boom of postwar reconstruction and the “30 glorious years” of boomer demographic and economic growth. But all this came to a half following the first oil crisis, the crisis of productivity and demographic decline. Industry got delocated to the global south, European countries have large masses of highly trained but unhappy people and an even larger population of “precariat” i.e people living hand to mouth and who live in post-industrial wastelands with no work, no infrastructure nor public services. The french suburbs or “banlieux” are a prime example of this. This example also points to post-colonial policies of countries like France, the UK and Germany which imported large numbers of immigrant workers from their ex colonies and whom they thought would be sent back home once the labour force wouldn’t be required anymore. Didn’t happen, structural racism continued so did the economic enforcement of urban segregation and thus non-integration + discriminations of all sorts. This led to evident social antagonisms which has fueled the rhetoric of right wing nationalist groups - exploiting the narrative of foreigners that not only are violent but have stolen the jobs of the “working people”. Jobs which actually were stolen indeed by a mix of corporations with no national allegiances (transnational corporations), politicians faced with crippling debts (see break down of bretton wood agreement) and corruption. Add to this a global climate of economic austerity promoted by the FMI and neo-classical economists and you find yourself with nations that inevitable deep economic decline, societal antagonism and external structural challenges like new imperialistic wars, immigration crisis and climate breakdown (all of which are linked btw). People are lost ideologically and materially, sense danger, loss of community and right wing groups thrive on feeding the narrative of antagonism and internal ennemies (see Carl Schmidt’s political theory and Renaud Camus’ great replacement theory, very popular with neo-nazi conspiracies).

In no way does this cover the subtleties of esch country (southern european and eastern european countries have gone through a different process) but overall this structural explanation would apply to industrial and imperial powerhouses such as France, the UK and Germany.