Hmm, well I respectfully disagree about the characters having "so much" depth in the game. But you're right, in the game at least the characters had, well, more than just one or two character traits. But in the anime we get... one or two character traits and not much more.

It's obvious why they wanted to animate the story. It was a popular game, and any anime adaptation - even a crappy one - means more $$$ in their pockets.