I can think of several glib theories each of which would have its adherents.  

Moral/Intellectual Decay is always a favourite;  if you are a rightwinger then the nation's moral fibre is being weakened by drugs, sex, permissive childrearing, gay and feminist activism, and probably sinister foreign influences;  if you are more of a lefty social critic then the nation's intellectual capacity and critical thinking skills are being dumbed down by braindead television, advertising, mindless video games, a decline in the quality of education and literacy, etc.  You then have either a morally or intellectually impaired public, which is not adequate to the task of participating in a functioning democracy.

Another theory would be that institutions naturally decay, that they have a life cycle just as monadic organisms have, and that the institutions of democracy are moribund in our time, have reached a kind of senescence and are about to perish of natural causes.  This theory has a hard time when it comes up against the Catholic Church :-)  but I do see a certain sense in it;  organisations (businesses, NGOs, think tanks, community groups) do tend to "lose the plot," lose their sense of mission or get infested with opportunists, over time, and require reform or refreshing or just plain starting over.  The Democratic Party might be a poster child for this theory.

Another theory:  capitalism is doing what it naturally does, which is concentrating more and more wealth and power into the hands of corporations and the individuals who own or control corporations.  This in turn naturally increases corruption, bribery, undue influence, media control and disinformation, etc., and this leads to the breakdown of democracy.  So it is not surprising that as the advanced capitalist nations' Gini numbers begin more and more to resemble mafiya-run Russia or some tinpot African dictatorship, the politics does likewise.

Another theory:  the great splurge of vulgarised privilege and luxury which the "advanced" nations wallowed in after WWII (the Great Fossil Fuel Party) offered such a glut of consumer goods, conveniences, entertainments and luxuries to "the people," and such a relentless marketing blitz to convince them to buy more and more of the stuff, that it changed the emotional tenor of life in the wealthy nations.  People became focussed exclusively on their individual existence and their collection of material goods, and ceased to think of public life and public probity as "goods" to be defended, watched, fascinated by, engaged with.  This is a variation on the Moral Decay theme with "sybaritic decadence of Rome" overtones:  imperial luxury destroyed the probity and backbone of Roman society and hence (with a rather long lag time) the Fall.

Yet another theory:  deliberate de-engineering of democracy by the elites, using economic instability and fearmongering (as discussed over on SusanG's diary about a Culture of Honour), i.e. democracy isn't just decaying, it's being actively demolished with shaped charges by people who know what they're doing.

All I know is, the voter turnouts in nations where voting is not mandatory are pathetic -- in countries that call themselves "democracies".  The I-Don't-Care Vote is nearly a majority in some countries.  This in itself is a symptom of crisis.  The lack of outrage at embezzlement, bribery and other malfeasance is another symptom (where are the mobs with tar and feathers?).  And imho the trend (esp in the US but also in Russia and other places) to "leader cult figures" and what we might call Hollywood Politics (political figures gaining power based on charisma, personality, good looks, the generation of a romantic mythology) is another bad sign.

Any other theories about the origins of the rot?  For I do think I smell rot, and as DoDo pointed out, in more than one country simultaneously.

The difference between theory and practise in practise ...