system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Human race

Why is humanity the most widespread race in the Galaxy?
We know EU ( Rakata and stuff ), we've seen a ton of other different races, but they are mostly  humanoid ( twileks, zabraks,  muun,  Sith, etc).

 With very rare exceptions like Yoda's species.

 Or the goddamn Gungans, who are humanoid enough , but I think they developed on Naboo on their own before humans arrived to conquer land surface ,  build their fabulous castles, procreate and elect a Sith Lord as their Senator.

Still - how and why did humanity become dominant in the galaxy far, far away?
gpt
Space cockroaches. Humans breed faster than many races, live longer than some, can interbreed with a few species. And most importantly us humans can survive damn near anywhere. Gungans need some water for life so tattoine and geonosis are off limits for long term habitation. Humans can survive on frozen wastes and burning rocks with minimal accommodation compared to most species. Toxic biomes and barren planets are a minor hindrance. 

But in legends the real secret humanity getting around was location. Humans developed on coruscant which is keenly located near dozens of major hyperlanes. They sent sleeper and generation ships out throughout the early years of galactic history. By the development of the early hyperdrives humans had already colonized dozens of worlds and had several offshoot species like the dathomir and chiss. They were relatively isolated from major species like the hutts so could advance their own hegemony without facing another species capable of matching them technologically for a few centuries if not millennia. 

Humanity got a major head start by being born at the galactic crossroads and having nothing but open real estate instead of neighbors. They found lost colonies and continued to settle world after world before running into another interstellar civilization. The rakatans also seemed to like the taste of humans and bred entire colonies throughout the galaxy to feed themselves on. 

This all created the perfect combination of biological, technological, and geographic location for humans to become the dominate species in population density and diversity.