It's a multitude of reasons people already mentioned. Getting good and then getting things done is hard, it takes time, and liberal people often act as gatekeepers. Having time and opportunities to break through requires a certain degree of income and connections. Something that upper classes have abundance of, and most of them are liberal. Demographics play a role too. There are more people living in urban areas, and that's where population swings left. Urbanites are more willing to invest into social programs, including ones dedicated to arts. They are richer too, so there is potential to make more money by pandering to them.
Then there is a theory of creativity crisis. For whatever reason, creative juices are not flowing the same as they used to within many contemporary societies. You could blame world wars shattering traditional value systems, communism, capitalism, atomized society, polarized social climate, environmental pollutants altering brain chemistry, mood stabilizing medication, good times-bad times theory, or anything in between. Quality of modern works is pretty poor all across the board for some reason. Contemporary right wing art is not good enough to make any impact. Left wing aligned people are creatively bankrupt too. They get to broadcast their trash only because they control media institutions.
I agree with this to a degree, but take it with a grain of salt. People always complained about how bad things "now" are, and how everything was better back in the day. Even ancient Greeks did it.
There is a way to do political literature without it becoming Randian slog.
You need to create fleshed out characters who struggle with or against the system, and tell their tale. That's what people like Huxley, Orwell, and Sinclair did in their best works. Their most acclaimed stories are good even though they are soaked in politics. These stories work because they do not stop to preach at the readers and characters are more than two dimensional props for the agenda. Of course that carries a risk of people learning a different lesson than what author might have intended. "The Jungle"'s impact is a good example. Sinclair's aim was to show people how immigrants and factory workers are being exploited. American public focused on poor sanitation standards of nation's meat industry instead.
I think the "liberal people control the publishing industry!" thing is largely a myth. Without fail the best selling political books in America are right wing rants written by people like Bill O'Rielly or Ann Coulter. Not only do books that are basically bitching about minorities and nothing else published, they make a lot of money. Now part of this is that republican organizations pad the numbers by buying thousands of copies of these books because they want to make it seem like people are reading them more than they are. But still, every grandpa in America has some book by a fox news personality or about how the gays are the reason we lost Vietnam lying around.
Though for what it's worth conservatives in the US have always despised the arts, if only because they think tax dollars going to anything but bombs is theft and therefore have no desire to actually support them. Culturally they tend to look down on anybody involved in anything creative with the possible exception of popular country musicians.
It's possible to write good political literature, I just think your average conservative has neither the inclination or the mindset to pull it off. If you want the pinnacle of conservative entertainment I'd argue its
Red Dawn. Now, I love Red Dawn. But it's an absolutely retarded movie with as much moral and intellectual significance as huffing spray paint in a Denny's parking lot. It's about the most simplistic, paranoid, basis for a film ever. The scenario is this: illegal immigrants secretly funded and armed by Cuba and the Sandinistas infiltrate the US by sneaking across the southern border, opening the gates to a soviet invasion and occupation of the American heartland. The first thing they do is take everybody's guns, illustrated by a great scene where a guy with a "from my cold dead hands" bumper sticker has his gun ripped from his cold dead hands. High school athletes proceed to massacre hundreds of highly trained Soviet and Cuban soldiers because America is just that badass.
How conservatives view the world is basically that. And it's impossible to portray it in art without it coming off like a parody of itself.
Progressives and left leaning ideologies are just as shallow.
That's why anything political tends to get confined to the background when you're telling a story.
And all if stems from gay bullshit like what Kalergi wrote. And you can tell because ideas like multiculturalism merging into a unified culture don't work and only sound good on a surface level. They never figured in needing things like assimilation and actively cutting parts of the foreign culture out so they can integrate into the host country. You now have people wanting the host country to be the one that makes the cuts and it just causes endless conflict because stuff like Islam doesn't mix with neo-liberal beliefs. They will claim it does but the moment that Afghanistan fell, gays were swinging from the signposts.
Which is why many dystopian settings are multicultural wastelands with warring factions.
If conservatives are Red Dawn then liberals are The West Wing. If red dawn is a paranoid, xenophobic, vision of a jingoistic nightmare then the west wing is a naive and idealistic mess that assumes every problem in society can be solved by appeals to reason and moderation. People are neither reasonable or moderate. They're fucking nuts. And the US government has always sucked and will always suck. It only ever does the right thing when people undermine it to the point it caves. Literally nothing good has ever come out of washington, it's come from regular people forcing washington's hand.
Anyway, I live in the most diverse place on the planet. It's fine. The existence of conflict does not negate the possibility of people living together in relative peace. In fact both usually exist at the same time.
I mentioned Ursula K Le Guinn up above. Most of her fiction is about people of vastly different cultural and political backgrounds having to find ways to live with each other. How do you create peace in a divided world? Generally her argument is that it's a never ending process, but a necessary one. There are no perfect solutions to anything, but there is always the possibility of change. And it's worth striving for.
Conservatives are incapable of that kind of ambiguity, so naturally their solution is just endless war of all against all for all eternity.