Like a lot of the seemingly random and arbitrary stuff in music, there's a good historical explanation. Western church music used a variety of rotations of the scale (a.k.a. modes, though that word also has a broader meaning) and it took a long time for C to become the "main" mode; for a long time it didn't even exist. See this StackExchange answer for more: http://music.stackexchange.com/a/895