How much of the delay in graduation that you observed was due to mainly financial reasons (e.g. had to work a lot of hours and therefore could take fewer courses per semester) versus mainly academic ones (e.g. poor high school preparation leading to difficulty handling full course loads)? Or was it that they often had both kinds of challenges (with the combination being worse than either alone)?
Some colleges have tried getting worse-prepared students to do the remedial courses during the pre-frosh summer, which would be less expensive for both the college and students than an extra semester, and also does not have as much of a marketing disadvantage as telling the student that the college expects him/her to need more than 4 (academic) years to graduate. However, students in financially stressed situations may not want to pass up earning money during the summer to help pay for school, even if the pre-frosh summer program were no cost to them.