What has changed is not so much that writers' work is influenced by their politics. As you said, that has always been the case. What has changed is that writers these days don't recognize that their work needs to be a good work of art first, and a way to express their views second. They lack any skill in subtlety or nuance, so the work becomes little more than a soapbox for the writer that is off-putting to all but the most ideologically aligned audience.
I like to use Star Trek in the 90s as a good example of what I mean. While there are episodes where the writers got preachy (they're only human I suppose), most of the time the writers were very careful to not openly take sides on the issues they raised. Even if you got the sense that the writer for an episode might feel a certain way about the topic, the characters wouldn't tell the audience how to feel. They didn't call other characters who disagreed with them names. They didn't just bully their way to victory in the story. The topics were treated as complicated issues where reasonable adults could disagree.
Compare that to shows/movies/books today. The writers treat the story primarily as a vehicle to express their opinions on issues. They have characters tell people "this is how a decent person behaves", with the understanding that the message is really meant for the audience. They have characters who agree with them call their opponents bigots or worse insults. They portray said opponents as villains or morons who only hold their beliefs because of how evil/stupid they are. They have the "good guys" run roughshod over anyone who disagrees with them, and they get to win despite their bad behavior. And often, the writers (and even other people involved like actors) will openly express their contempt for their audience when speaking about the work. They pick fights where none needed to happen, saying stuff like "if you don't like this then I don't want you as an audience member anyway". They are, in short, bad writers who don't have the skill to successfully let their social views influence their work.
The result of all this is that these writers don't succeed at persuading anyone. In years past writers could actually make progress on advancing the things they believed in because they had the wisdom to not openly preach to people and call them names. They respected people enough to let them draw their own conclusions, and as a result were successful. But writers today aren't good enough to persuade people to continue breathing, let alone something more controversial than that.
There is also an uptick in how much politics get forced into art, with people trying to claim "everything is political" and the like. But that isn't nearly as big a factor as how bad today's artists are at using political themes in their work.
I like to use Star Trek in the 90s as a good example of what I mean. While there are episodes where the writers got preachy (they're only human I suppose), most of the time the writers were very careful to not openly take sides on the issues they raised. Even if you got the sense that the writer for an episode might feel a certain way about the topic, the characters wouldn't tell the audience how to feel. They didn't call other characters who disagreed with them names. They didn't just bully their way to victory in the story. The topics were treated as complicated issues where reasonable adults could disagree.
Compare that to shows/movies/books today. The writers treat the story primarily as a vehicle to express their opinions on issues. They have characters tell people "this is how a decent person behaves", with the understanding that the message is really meant for the audience. They have characters who agree with them call their opponents bigots or worse insults. They portray said opponents as villains or morons who only hold their beliefs because of how evil/stupid they are. They have the "good guys" run roughshod over anyone who disagrees with them, and they get to win despite their bad behavior. And often, the writers (and even other people involved like actors) will openly express their contempt for their audience when speaking about the work. They pick fights where none needed to happen, saying stuff like "if you don't like this then I don't want you as an audience member anyway". They are, in short, bad writers who don't have the skill to successfully let their social views influence their work.
The result of all this is that these writers don't succeed at persuading anyone. In years past writers could actually make progress on advancing the things they believed in because they had the wisdom to not openly preach to people and call them names. They respected people enough to let them draw their own conclusions, and as a result were successful. But writers today aren't good enough to persuade people to continue breathing, let alone something more controversial than that.
There is also an uptick in how much politics get forced into art, with people trying to claim "everything is political" and the like. But that isn't nearly as big a factor as how bad today's artists are at using political themes in their work.