Why is Horde always more populated?

Looking at sites like WoW Progress (which I know isn't 100% accurate but still), Horde has two servers that are over 30k people, while Alliance's biggest server is 26k (second biggest is 17k).

Alliance has three servers that are 14-17k, while Horde has nearly 10 servers above 14k.

Why is Horde way, way more popular? Seems like 70% of the population is Horde. And it's been that way for as long as I can remember.