The reason is fairly simple. A human is a very solid middle ground. The weird alien shit usually has very little common ground across your target audience (if you decide to pick one race only), because for everyone that adores the butt-ugly fish race of admiral ackbar, 10 people wouldn't want to run around as the hideous fish monster for more than 2 seconds. Personally I absolutely can't stand the aliens with the fat-flaps/tentacles on their head like Ahsoka and rather play a boring-ass human than someone elses feish dream.
That they can't add 2 voice actors and a rudimentary character creator in a game in 2019 with the budget of a star wars game is another issue entirely. That part is just sad, since I doubt the character we will be getting here will have enough deph to warrant just be that character.
As for the trailer: It was a gameplay and skill showcase. I think it gave enough context to see what they are going for. Not sure why some people seem to lose their shit over it.
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I think the closest to an RPG it could get would be something like jedi knight (iirc) where you could make some builds depending on your taste and did a little bit of light talking with NPCs here and there, but certainly not a kotor.