There's slightly less to get fired for as a prelim. In general, interns get fired for lack of clinical skills, or professionalism / communication problems. If you have the latter as a prelim, you're (perhaps) more likely to get fired -- if you are fired from the program, it won't matter the next year. If it's the former and you can't do the intern job, then you get fired just the same. But if you can do the intern job but don't appear to have the skills to handle the resident job (i.e. can deliver care with direct supervision but not with indirect supervision and managing an intern), then as a prelim that's fine since you'll never supervise IM interns. This situation is probably the most common, hence prelims get fired less often.