Such a system would likely help improve quality, it's just that it'd also be extremely difficult (and potentially costly/time consuming) to implement. After all

Such a system would likely help improve quality, it's just that it'd also be extremely difficult (and potentially costly/time consuming) to implement. After all, likely importance and accuracy are also (to some degree) community specific as well as relevance. A forum based on politics might consider a Donald Trump memo leak as the most important submissions whereas one about video game speedruns might consider a TAS video as theirs. A forum or site that supports a certain candidate might (unfortunately) want articles that support their preconceptions voted up more. And heck, even the way said factors are weighted would be context dependent. A news site striving for accuracy or a skeptics community might want to focus more on the submission's accuracy or literacy while a subreddit may want to focus on the likely importance of said submission. It's an interesting idea, and I would love to see a system which could weight factors like this in a way that's easy to tailor for different communities. But when you consider the time and effort needed for such a system, I can definitely see why most just go with typical vote counts for ranking stuff.