The simple reason is I've been collecting ninja/stealth games/mods.
I have a more complicated reason, which is I'm thinking about a ninja mod for the next Unreal - not a traditional TC mod, but something more like a federation of related mods - weapons, tools, models, and maps that happen to fit with the ninja theme, and that other people could repurpose in their own mods - with appropriate credit to the original authors. Does a UT mod fit in? Maybe, maybe not, but it would be interesting anyway.