First, experience has taught me that stories are usually written with the assumption of a human character, so I tend to prefer playing a human simply to dodge immersion-breaking moments of forced ignorance about my non-human protagonist's own culture ... and because only having significant cultural/physical/social status differences acknowledged once in a blue moon makes playing a non-human in a human-dominated setting feel pointless.
Second, I really wanted to roleplay a devout but open-minded Andrastian, so the Trevelyan background was the perfect fit even before the game smacked me over the head with the delightful "Herald of Andraste" business. Engaging with the implications of that for my character -- isn't it blasphemy to even hope it's true? what if it is true and I fail not only my people but also a mandate from my god? etc. -- fascinated me, even though the game failed to really bring it to life.
Third, I found the non-humans' reason to be at the Conclave pretty dubious, because it makes zero sense to me that a bunch of random and potentially hostile/criminal yokels would even be allowed within a three-mile radius of this colossally important and high-risk event, nevermind within immediate reach of the Divine herself. Also, the entire mage/templar war and future of the Chantry issue speaks much more to a human character right off the bat, even before the "Herald of Andraste" issue comes into play. I like to play characters who make the most sense to me in terms of feeling connected to the story -- in a dwarf-focused story, I'd most likely jump on a dwarf character.
Fourth, while I dislike the scrawny female human model, at least it doesn't look totally wrong like the "starving babyteen" elves or the "basically short humans but still-cute because that's apparently the most important thing about a woman" dwarves. (At least the male dwarves get a more distinct look with kind of cruder, heavier features, big noses and all.)