I'd argue that the real reason there are no XBox emulators is because nobody with the requisite skills cares enough to bother to reverse-engineer it and write an emulator.
Since there are very few exclusives, there's little demand for it to be emulated in the first place. And from what I understand, the XBox isn't fondly-remembered by geeks. The XBox was the console of choice for people who liked to make fun of geeks and nerds, and as such most people who have the skills and the desire to write emulators (i.e. geeks and nerds) have a pretty strong aversion to the XBox. That the most popular games for the XBox weren't the kind of games that were enjoyed by nerds doesn't help, either. There's very little overlap between emulator programmers and people who care about the original Halo or old Madden games.
Since there are very few exclusives, there's little demand for it to be emulated in the first place. And from what I understand, the XBox isn't fondly-remembered by geeks. The XBox was the console of choice for people who liked to make fun of geeks and nerds, and as such most people who have the skills and the desire to write emulators (i.e. geeks and nerds) have a pretty strong aversion to the XBox. That the most popular games for the XBox weren't the kind of games that were enjoyed by nerds doesn't help, either. There's very little overlap between emulator programmers and people who care about the original Halo or old Madden games.