Because I like evil and darkness, not you prissy little mages. DOn't expect a pally of all classes to understand.
And given the later comments in your post, I pray you're trolling.
Although indeed my post was all in good fun, there is one thing that seems to be holding true. People prefer the classes that have more of a noble or honorable feel to them. Rogues are a less played class, but there's the whole honor among thieves thing going on. Death Knights are an outlier, but they also start at a much higher level, so they are more accessible to most people as an alt. They also didn't choose to be what they are, but were instead forced into it. The entire theme around warlocks is that they are dark, emo,
Emo refers to emotional, what does being a Warlock have to do with that? It helps to know the meaning of words before you use them. Because it deals with dark themes? So are Rouges, does that mean Rouges are emo too?
On the whole people are attracted to noble themes, but making the class more accessible isn't giving it artificial bonuses. Furthermore, Paladins are the only stereotypically good class outside of Priests and Shaman. Mages, warriors, and, hunters are largely neutral. They are typically mixed in the roles they portray as opposed to Paladins and Presist with the former of, which tending to be very strong. DK's were forced into being evil, but they still use necromancy and blood magic. They just happen to be much easier to use than locks and the return for using them is larger.
People are also driven by appealing traits, and if someone sees something appealing they will flock to it. This is why belfs are the most played Horde race. Being considered the "bad guy" isn't as massive a turnoff if the bad guy looks appealing and powerful.