And Hearts of Iron also has far more interesting and detailed mechanics than EUIV.
It has far detailed mechanics because it is incredibly more specific in time period and historicity - it is less than 20 years as opposed to freakin 380 years of EU4!!! Add to this the fact that HoI world is industrialized and 'westernized' (all significant powers) and there are mostly the same mechanics for Latin American and East Asian countries. Autocracy or democracy, oil, planes, tanks, copying western model to lesser or bogger degree etc.
Meanwhile in 1444 AD North America, Andes, Mesoamerica, Western Europe, Russia, Hordes, Islam, Subsaharan Africa, India, Indochina, China and Japan were all extremely, extremely different from each other, there is no common ground to simulate here - if someone wanted to represent each of those areas in DETAILS, he would need to remake base game for each of them.
There was more difference between governing models of Russia and neighboring Commonwealth from this period than between Vic2 and HoI4 government mechanics.