First, a fun fact: 47% of all Death Knights being played as the primary character are level 80. 77% are 70+. Levels below that are just a tiny number of people at each level, with one odd data point – 7% of primary death knights are at level 64 exactly. Without being a death knight myself, I can't venture a guess as to any possible causes. I look forward to the comment threads.
Humans and blood elves, what a surprise. In raw numbers, those are the two most popular races
as a whole, so the fact that they are the two most popular races for the Death Knight isn't a surprise. Even when people are trying to play something as tough and powerful and cool as a DEATH KNIGHT, they still want to be pretty.
Admittedly, people seem to care less about being pretty death knights as opposed to other classes – human characters are 38% of the total in the game, and as you can see, DKs are only 24% of the total.
I am told that humans are a popular choice in part because there's less faction grinding required for the widest possible access to in game goodies. This makes sense, given that the class started life as an alt for most people. And it would explain why the proportion of human characters of the new class is bigger than any other class. One problem with that theory - there is no equivalent faction explanation for Horde that I'm aware of, and the blood elf proportion is even bigger than the human wedge of the pie.
So, I'm going back to the pretty theory. I said back in October that even though everyone tries the freak races, they tend to gravitate to human before long... and blood elves are as human as the Horde gets in terms of appearance. This has nothing to do with gender – male players are less concerned than female players about being pretty, in that there are almost no female players with orc characters and only a handful more playing dwarves. But men still want to look good.