The better postbacs are structured programs that can be done within a year. They typically have superior advising than the vast majority of most pre-med offices.
Additionally, these postbacs are not actually geared towards people who graduate straight from undergrad, but rather people who have taken a few years in other careers. Most of the people in these programs didn't decide upon medicine until well after graduation.
As for GPA repair programs, I'm not sure why, but I imagine if you didn't succeed in the same environment the first time around, it may not be worth the risk for some people to try again.