it's less because of the way they came to power and more because it's the base of their power. dictators have to *maintain* their power because they're dictators. they need to keep the military on their side and suppress anyone against them. they need to 'justify' their rule, basically. monarchs, on the other hand, are the justification in on itself. they still need support, obviously, but there's zero question as to why they are in charge. of course, there are a few exceptions for dictatorships that aren't just a military junta and actually do something, but those are usually the result of an particularly talented leader, like franco, or even getulio vargas in my own country.
Brazil