Ignoring any bias in the demographic itself, it seems to me that it's just harder to write right-wing sci-fi.
It's easy to write about an enemy that has all of the money, power, and control, providing a convenient struggle for the protagonist. It's much more difficult to write (and thus, more rare to find) a good story where the enemy is given the same circumstances as the protagonist, and both are given the same life choices. Sure, you can say that the antagonist was given too much freedom... but then you have to establish why he chose that way, and if you try to use any variation on "because he's evil", your story goes from being a thought-provoking philosophical adventure to being a heavy-handed morality essay.
You could more generally make a strong leftist state be the enemy. That's the dystopian road blazed by sci-fi in the 1960s, and several movies since the 1970s, notably Logan's Run and Soylent Green. It's a time-honored genre, but that's also the problem. It's old. Dystopian fiction dates back a few centuries, and combining it with science is hardly groundbreaking.
For a while, a popular trend was to base such stories on real people and events, who could be suitably framed for conflict while keeping their political slant. Of course, the reading audience quickly grew tired of every Nazi and Soviet alternate-history piece, and those have waned in recent years.
To more directly address your point, consider the alternative: Writing left-wing sci-fi is easy.
The plot is simple: An underdog wants freedom for himself against the oppressive regime of the evil overlord, who had freedom and used it to oppress others. What makes the underdog different is that he will stand for justice for everyone, show kindness to everyone the audience could identify with, and never miss a chance to help others. Relying on teamwork and everyone's unique (identifiable and presence-justifying) abilities, the protagonist establishes a utopian foothold, where all of the characters that the audience identifies with are loved and cared for.
In fiction, that plot is sufficient for a story. In reality, things are much more complicated. What happens if one of the protagonist's allies was really only following because his girlfriend was? What if the rules of the protagonist's new state really screw some of the wealthier folks? What if the protagonist himself is genuinely a right-wing capitalist who just wants to make money and retire in obscurity?
You're right - Political strife isn't where really good fiction comes from. The best sci-fi works are ones where every character has their own motivations, and they don't boil down to "be good" or "be evil". Rather, they reduce to things like "sleep safely", "get back to stability" or "avoid the consequences of a mistake". One particularly good sci-fi space opera work, itself nominated a few times for Hugo awards, has spent nearly two decades dealing with the indirect results of a mistake. Characters have come and gone through the series, and politics has been an issue, but by that time every character had their own long-established reasons to hold their preferences. At no point was the story ever purely about morality, even when the "definitely evil" characters were introduced - they eventually got their own motivations.
Those deep-rooted, long stories that take the time to establish characters and motivation are great sci-fi, and can avoid bias toward either political slant.
That's hard, though.