Have you not seen the huge amount of Half-Life mods? There're thousands. I'm not certain but I think the code was released at the same time the game was, or very soon after. You've heard of Counter-Strike? Never would've happened if they'd not released the source code. That takes me back, playing Beta 1 of Counterstrike. They did the same with Half-Life 2. Code was released very soon after and thousands of mods were made. Unless there's some specific problem, Half-Life could've been ported to Xbox around the same time Quake was. I don't know, the Half-Life community is 99.9% PC based, so maybe there wasn't interest in doing it.