I've heard that the more Card wrote, the more he tried to infuse his writings with his Mormon faith, which is why it comes of preachy. I could be wrong about that, though. Ender's Game and Speaker for the dead both came off as pretty secular to me, though clearly Speaker had more of a preachy edge to it. In Ender he was really just trying to set up a character and not preach anything, and this seemed to work a lot better.