Because mIRC is coded by one person, and because that one person doesn't want to spend his free time porting it? Sounds simple enough to me. And don't even bother with the 'well why not make it open source', I think anyone with half a mind can figure out why that won't happen.
Or maybe thats not the reason at all. Maybe Khaled just plain doesn't want to, maybe he doesn't like Linux, maybe he's got a pact with Bill Gates, who knows? Bottom line, it's been said 1000 times that it won't happen. I doubt that'll change anytime soon, if it does, you'll know it when it's announced.