The alliance has some very hardcore, dedicated players and I see these same names over and over again in most games I play. I think they play like 12+ hours a day, because I can see them whenever I queue. I don't think the horde really has any players that are that hardcore and who also like to pvp. The only two like that that come to mind are Kharino and Tierra, but they stopped. At least during North American play times, the horde just doesn't have many people period.
But it really comes down to the way the alliance plays. It might not appear that different from first glance, but (for example) its hard to find an alliance player by themselves in WSG. They work together, support each other. Most horde players are glad to run off by themselves and charge a group of 3+ alliance and then complain that their fellow players are awful. When I grab the flag, I have 4 alliance on me instantly, and zero horde coming to support. When the alliance get our flag, they have 3-4 people coming to defend and support right away, and maybe 1 or 2 solo people from the horde try to return. As a healer, I just try to find a group of 2-3 horde together that I can go heal and support. I usually can't find that.