Its becuase people of some countries wanted that each and every person of that country should be equal, as they wanted that each and every person, whether a foreigner or born in that country if wanted to be a citizen of that country, they should be. And especially because in ancient times, the usefulness and strength of woman was not being recognised by people, and many people had the wrong idea that men are superior to women and should always be discriminated...this was also the condition with the unequality on the basis of the ranks of people and the property they owned. the rich people were always supported, the middle class people,were dicriminated little, but weren't recognised even though they were great minds, and the poor people were highly dicriminated. So, many countries adopted democracy.
i hope i helped..
