Humans are the most popular because the other races aren't shown as much. It happens every game.
---Origins---
Nobles can become queens or kings.
Mages revisit the Circle regardless of race. Magic being astounding.
D. Nobles come back to politics and then their race becomes diminished in influence otherwise.
D. Commoners get the same treatment in different ways.
Dalish got the worst origin of the bunch.
City Elves were pretty damned interesting all things given though. But the sound of being a warrior in the forest sounds more appealing to an elf.
And Ferelden was prominently human. Redcliffe, Ostagar, Denerim, Soldier's Peak, Cousland Castle, Lothering, The Circle. You had human representatives and human culture taking the for front because that's the prominent role being displayed.
Orzammar is a Dwarven City, one of the few that remains. They keep to themselves.
Elves live in some forests, or in the slums called Alienages.
But royals, nobility, the power to change nations? Lies in the hands of humans. Except for Par Vollen which is this mystical Qunari land we'll never see.
Also look at the party ratios.
1 Dog, 1 Golem, 1 Elf, 1 Dwarf, 1 Qunari, 5 Humans (Including Loghain)
Our first game gives us loads of humans to show us the incredible culture and background they made. Leliana shows the Chantry and Orlais, Alistair shows Ferelden, the Chantry, and the Grey Wardens. Morrigan and Wynne give details on Apostates and the Circle respectively. Loghain shows us Fereldan and it's History.
While Elves, Dwarves, and Qunari get poorly represented. Sten, Zevran, and Oghren were their own windows into their cultures, with Shale being optional content that added a bit to Dwarven culture. Let's add that only one of the four romances wasn't human.
---Awakening---
2 Dwarves, 1 Elf, 2 Humans, 1 Spirit.
You'd think it'd be even between Dwarves and Humans now, but it's not.
Nate shows Nobles.
Anders shows a different side to Apostates that isn't as extreme as Morrigan was.
Justice is our first encounter with a Spirit.
Velanna shows Dalish and their approach on Magic to an extent.
Sigrun gives us details about Commoners in her society and the Legion of the Dead.
But Oghren? He doesn't offer much new in terms of letting us know Dwarves. We get acquainted with his character, and he develops there, but little knowledge on Surface Dwarf life or any prejudice he might face, or disorientation with being on the Surface and living there.
We get Amaranthine, Wending Woods which didn't have any Dalish Clan to speak of, Black Marsh and Vigil's Keep. All immersed in Human settings and that's what we interact with.
Kal'Hirol gave us some mighty information regarding Dwarves and potentially made them more appealing as a result, but it's the only huge venture Awakening offers in a human-centric game.
---DA:O DLC---
Golems of Amgarrak was interesting in regards to Dwarves.
Witch Hunt offered many previous places to explore but brought to light new information that was engaging.
Leliana's Song was mainly revolved on Ferelden-Orlesian politicking. Human and all that.
---DA2 and DLC---
2 to 3 Elves, 1 Dwarf, 3 to 6 Humans
Romance-wise it was split evenly between humans and elves mostly. Anders and Isabela to Fenris and Merrill. Tallis and Sebastian were also up for some DLC flirting/romance too.
Isabela gives Rivanni some background and shows us human raiders.
Anders shifts and offers us exploration on Mages being oppressed.
Being Anders complete opposite, Fenris shows us People being oppressed by Mages.
Bethany shows how a regular run-away mage is probably like, if sheltered by their family.
Merill shows Blood Magic and the Dalish reaction to it. She's also the first Blood Mage in canon to be apart of the team.
Varric gives us a suave look into Surface Dwarf life and how they manage with humans and the like.
Aveline and Carver offer more exposition on Fereldan and the transition to Kirkwall. And the act of being a prude sometimes.
Sebastian gives us Starkhaven and more Human Nobility to take account of.
Tallis explores the Qun and lets us know more about that.
And DA2 did a good job all around giving good information on all races. Sundermount, the Alienage, and the Elves that sided with the Qunari, Mark of the Assassin. It expanded Elven Lore greatly. Varric and the Cartel he ran around Kirkwall alongside the Deep Roads Expedition showcased Dwarven interest. And Act 2 was the spotlight for anything and all Qunari and one of the highlights of the game. Other human nations also got some story thrown their way.
Even so this all takes place in Kirkwall. A human city, with human leaders. But Orsino being an elf was interesting but never touched upon. A leader, and Elven. It had lots of potential as someone with prominent power, even if imprisoned.
---DA:I---
From the looks of it; 1 Dwarf, 1 Spirit, 1 Qunari, 2 Elves, and 4 Humans.
More diverse cast with at least one representative where it counts. Loving the potential this game can have.
---Ending---
Humans are the prominent race and are always at the forefront. DA2 even forced your hand to be a human.
Thus more opportunities tend to be available if you take a human character. Humans are even the main thing focused on in most of the books released on DA too. (The Masked Empire being a notable exception.)
I have a feeling that playing as the other races this time around will feel more rewarding.
[And hope that if race mechanics are successful in DA:I, that they are implemented in the next ME. I adore their races compared to DA's races personally.]