Democracy requires a demos, ie for the vast majority to share a set of common fundamental values.
Without it you end up with multiple "tribes" who vote tribally.
In both the UK and the US, there was such a common set of values, the tenets of Protestant Christianity, which were also shared enough with the other principal faiths (Catholic Christianity, Islam and Judaism) and with those of marginal or no faith to (outside Northern Ireland) form a cohesive Demos.
It is the rejection since the 1960s of those values by the "managerial" classes, and their attempt to replace it with a secular society based on secular notions of equality and human rights, which has fractured the demos and Trump and Brexit are the first tangible fruits of reaction against this. I very much doubt they will be the last.