Note that much attrition of college students happens in frosh year, when students new to college have difficulty with the transition from the highly supervised environment of high school to the much more self-directed environment of college (even at colleges that have lots of student services and an integrated residential experience like many highly selective private colleges). Transfer students have already succeeded at college (often at colleges with far fewer resources for student services in the case of community colleges), so the transition to college that some frosh have difficulty with is not an issue for them.