This is because a large part of the social and political decline we see in the United States is seen because it became a democracy, Democracies are very similar to republics to the point most people get them confused. But the 'franchise' (aka voting) is available to everyone in the country, while I would agree this sounds like a wonderful idea and a way younger me would certainly agree, age and reality as I've seen it has proven my current belief to be undeniably true at least to me.
Lots of people especially in high crime/drug use cities such as California are simply filled with selfish people who care more about themselves then the country or other people, and they use they're voting power to bend local laws and welfare to benefit them and people just like them. That's why in very blue states we see criminals practically legalize robbing people because self defence and guns are illegal or restricted to the point of uselessness, this along of course with the cripplingly bloated welfare state which is slowly killing the entire country. This is the clear and observable events which happen to every blue city and are a good indicator of giving people lack responsibility voting power.