I believe it has to do with linkages to their schools, I think some even let you transfer your classes over to their medical school. Pre-meds who do the DO SMPs can also apply to MD schools. I guess some people want to earn a degree at the end of their post-bac, which DO SMP may give whereas a DIY postbac will not get you that. Just my opinion, but I guess there are pro's and cons to both sides, a DIY is definitely a cheaper way to boost your GPA.