Technology advancing, particularly communications.
Firstly, bear in mind some form of monarchy was basically the most common and successful government for tens of thousands of years. The current crop of governments could very easily be a brief blip historically compared to the incredible run monarchy had.
Monarchy is a very successful system where communications are poor. The monarch tends to act as high general, high judge, and in some cases high priest. The King was responsible to reigning in abusive nobles and provided the last resort for the peasantry to request relief from injustice. The King lead forces in battle and was generally expected to be able to fight himself. The King, even if not a priestly figure outright, generally had a significant role in religious services, making shows of piety, and being a moral center.
Note that there's some significant holdovers even into modern systems, the US President, f'rex, has the right to grant pardons (high judge) and is the Commander of all armed forces (high general). There's also a significant expectation (generally a disappointed one) that the President will be of good moral character.
Modern communications negate much of that, however. When a case is dubious it becomes known immediately to the people and it's tried in the court of public opinion, there's no expectation that the President of the United States needs to personally investigate and hand out pardons if injustice is done in the courts. When there's combat the President has no expectation of leading forces or fighting, he transmits his orders via radio from his office instead. And as for moral character... that increasingly became a matter of political scheming and endless mud-slinging campaigns rather than anything to do with true morals. In no small part, that too was a result of mass communications making it easier to lay each other's faults bare to the public eye rather than allowing the monarch any privacy and dignity of office.