My personal theory for the popularity of the Witcher type "grey morality" / "no alignment" is the U.S.' endless wars. Soldiers are fighting a war that started before they were born and no one seems to know what the greater principle is -- outside of some generalized sense of patriotism or the like. So a character like Geralt that just does the job they were given and who has a limited ability to influence the larger events taking place in the world resonates in a way that character that CHARNAME with their ability to bend the universe through the application of willpower does not.
This is an inaccurate understanding of his story. The point of Geralt's personality is that he is forced to deal with situations where he has a choice between two shades of grey (sometimes there is a hidden third option but sometimes there isn't). He tries to be a heartless mercenary, killing monsters only for the money and and staying out of human matters. However, while a job killing evil monsters is theoretically supposed to be simple, it isn't, because some "monsters" aren't evil. Meanwhile, on human matters, he regularly has to grapple with the consequences of failing to choose the lesser evil, until eventually he learns the importance of making the lesser evil choice
(but carefully). Comparing it to US politics, thematically you can see it as essentially roasting anyone who sat out of the 2016 US election because "both sides are bad".
This arc is playing out in the TV series, but it's pretty much fully resolved before the events of The Witcher 3. By that stage, the idea that he's still trying to act like a neutral uncaring jerk is just used as a gag because all his friends know it's bullshit and in reality he's got a big heart.
(And yes, as Sozz said the political atmosphere The Witcher is largely based on early 20th century World War era.)