Lots of reasons.
1. If you are an actual fan of warcraft and played WC3, Horde has Sylvanas and Thrall. You can't join Arthas or Illidan, who you also played as. Alliance has Tyrande but she's useless and doesn't do anything.
2. Alliance has the pretty races and that would attract non-gamers to that side, creating an uncertain population pool for raiding and pvp.
3. Blizzard helped screw this up because the story centers around the Horde when they're not uniting against a common enemy. Thrall is the central figure of cata. Then Garrosh. Now Sylvanas. Khadgar? Yeah he's human but the Kirin Tor is neutral. Alliance is just "there."
Mostly it's 1. Most people don't care about the lore, except for Warcraft 3 and Frozen Throne because they were so well executed.