But limits in costuming/special effects technology was likely not the main reason for the main characters being human. Simply put—people can relate to other people a lot better than they can to something that isn't human. At least, that's my belief. By having humans as the main characters, you can project yourself into their place and understand the things they're going through—and Imagine what it would be like if you had to. I feel that a lot of that would be lost if, say, Han Solo had been a lizard-guy (it's always some kind of big lizard!).
Strangely enough, in my own campaign this turned out to be true as well. Without any guidance from me, almost all of my players chose to play human (or at the very least near-human) characters. Ruukhan the wookiee was the exception to this, but after a while, his player DID switch to a human character (Horatio). In the case of Bob the Tusken, I sat down with him prior to the game and we both worked out essentially what Tusken's were—humans (or near-humans) who wore elaborate desert garb. That's it. No tusks or scales or anything else. Just humans who evolved to live in the harsh desert of Tatooine.
Now, I'm not sure if this human-centric thing was because of the way I ran my game or not. Maybe I was secretly prejudiced against aliens, but I don't think so. I tried to keep things interesting for ALL my characters. What it seemed like to me is that folks enjoyed playing them for a while, but maybe couldn't relate as strongly to them as they did to human characters? I don't know. Just my best guess. Todd, if you're reading, I'd be curious to know.
In any case, it is this relatability factor that keeps humanity on the top of my list of favorite Star Wars races. That's not to say I don't enjoy playing lots of Alien NPCs—they make for great contrast and add a lot of character to the game—but they don't quite hit home as directly a plain, boring humans.
Oh, and p.s. — one of my biggest pet peeves in Star Wars gaming (and I really only ran into this online) was the tendency for 'twinks' to choose races based solely upon stats and special abilities, and no real desire to play as an alien. I can't tell you how many goobers wanted to play a Barabel or Defel or Noghri or Esoomian or Togorian or even Gamorrean simply because they thought it would be a 'super character'. But like I said, thankfully, that never happened in my tabletop game.