In the past, all or most of the power was held in the hands of the monarchy and the hereditary nobility. Nowadays, however, places that still have a monarchy are largely governed by an assembly of elected officials who do not necessarily come from noble stock. What caused the shift from the former to the latter? What sort of societal pressures existed that caused the nobility to willingly cede the powers of governance?
From a socio-economic point of view rather than a political one, in Britain at least the rise of Empire and increasing industrialisation lead to an economic shift from all the wealth coming from the land (owned by the nobility) to the wealth coming from commerce (businessmen/middle class). As agriculture declined in profitability so did the estates of the landed gentry, and so many were either forced to sell their land off to their tenants in order to maintain their large manors, marry into the nouveau riche business classes, or simply give up their estates altogether and live modestly. With traditional voting rights staked to property until the 20th century, the waning of the aristocracy gave more political influence to the lower classes, however this was a process spanning a couple of hundred years