This answer states that 42.6% of all beings in the galaxy are humans - a staggering percentage, considering that the runner-up, the Twi'leks, constitute only about 3%.
This is an astounding statistical discrepancy, which means, of course, that it is highly unlikely that it is a coincidence. So why are humans so plentiful?
I wrote that answer, or at least assembled it, and I think you're overlooking a couple of important caveats which I included in the answer:
Now, keep in mind that although this list was compiled in as scientific a manner as possible, it doesn't necessarily reflect the actual demographics of the galaxy. It reflects the demographics of characters prominent/popular enough to have both their own names and their own Wookieepedia pages.
And:
This post suggests that there are between 100 quadrillion and 400 quadrillion sentient beings in the galaxy. The problem with relying on Wookieepedia for our data is suddenly all too clear: We know that humans are the dominant sentient species in the galaxy; we know that there are at least 100,000,000,000,000,000 sentient beings in the galaxy, and a large portion of these must be humans; yet our figures list only 9,467 humans, less than one billionth of one percent of the total population.
I'll repeat that, because it is very important: Our data about humans is based on a sample size of
So what the answer to that question is actually saying is this: Of all the characters who are prominent enough, and popular enough, to have their own Wookieepedia pages, 42.6% are humans. This doesn't reflect the actual demographics of the galaxy so much as it reflects the fact that humans like human characters more than they like non-human characters. I would venture to say that Twi'leks are the second most common race because their women folk are pretty.
By way of analogy, if we based our calculations on the contents of ancient historical records, we would probably come to the conclusion that everyone in the ancient world was a king, queen, prince, princess, priest, bishop, senator, etc., and average people didn't exist.
Or, as The Hitchhiker's Guide to the Galaxy puts it:
"Many men of course became extremely rich, but this was perfectly natural and nothing to be ashamed of, because no one was really poor - at least no one worth speaking of."
- Thanks to Daniel Roseman and Andrew Thompson for mentioning this quote
The out-for-universe answer to the question of why there are so many humans in the Star Wars series is, as Ian Auld points out in his comment above, a matter of pragmatism. Humans exist in the real world. The vast majority of actors are human. You don't need to apply expensive and complicated prosthetics to make actors look human. The audience is human. Humans relate to other humans. Therefore, it makes sense to cast as many humans in the movies as possible.
The same idea applies to the language spoken by most of the characters: English. It is unlikely that aliens living in a Galaxy Far, Far Away would speak English, obviously, but the original target demographic for the Star Wars movies was largely American. Faced with the choice of either (1) forcing the audience to read subtitles throughout the films, or (2) having most of the characters speak English, George Lucas wisely opted for the latter.
So the simple answer is that human audiences prefer human characters, and English speaking audiences prefer movies with English dialogue.
Note: I would suspect that humans are indeed the most prominent sentient species in the Galaxy Far, Far Away, but they almost certainly don't represent 42.6% of the population. Unfortunately, there doesn't seem to be any canonical information about the demographics of the galaxy, so we don't really know for sure.
