Why did the growing influence of the middle strata lead to the further democratization of Western countries?

As you know, democracy is based on equality. Democracy is precisely this political movement that is the main one in the overwhelming majority of Western countries. Their ideology is based on the fact that the middle stratum of the population is the support of the entire state, because when it exists, there is no strong contrast between the rich and people below the poverty line, as in the countries of Eastern Europe.