Well, first off, you've got the plot of silent Hill 2 confused with the plot of the first game. Harry's wife sends him no letters.
Also, according to Chris Gans:
"I think when Roger Avary and Nickolas Boukhrief and I started to work on the project we assumed, because we are all fans of the game, that we would start writing with Harry Mason as our lead.
It quickly became clear however Harry never acted like a masculine character. He was constantly dizzy, fainting, talking to himself, screaming and in fact was very vulnerable. We didn't want to betray the nature of the game by changing the character's feelings and motivations, so we felt it was better to change to a female protagonist and retain all those important qualities. I don't want people to think that I have been "politically correct" because we changed Harry into Rose. There is no political correctness in Silent Hill.
It's important to mention that we were conscious that allowing the Gamers to have to face a new character would help in the transition from Silent Hill the Game to Silent Hill the Movie."
You can read info like this on Gans' Blog.
http://www.sonypictures.com/movies/silenthill/productiondiary/index.php
Ah ok, cool. Thanks for clearing that up.
And yeah haha, I didn't get the two mixed up, oops. It's been forever since I've played Silent Hill titles.