"Western democracies" seems a euphemism for "capitalist democracies" or "liberal democracies." Either way, it refers to a mix of two different phenomena, i.e., an exploitative social system (capitalism) and democracy.
I think that Marx was wrong if he thought that capitalism automatically produced democracy. Instead, it would be more accurate to say that it produces small-r republicanism, i.e., either a republic of the the rich or some sort of constitutional monarchy. (In times of strong overt class struggle that threatened the _status quo_, even this has been jettisoned in favor of military dictatorship, fascism, etc.)
What produces democracy in a class society is popular struggle from below. Because the popular struggle has not been completely successful, the democracy has been limited.
In the US, there was a certain amount democracy from the start, because capitalism was undeveloped and petty property was very important (though this was couteracted by the existence of slave property). This democracy increased due to popular struggle, of small property owners, of workers, of women, of minorities, etc.
(off-list for a few days) -- Jim Devine "Science is the belief in the ignorance of the experts" -- Richard Feynman