I read this article from the Fiscal Times. It's not the most credible source, but it was interesting to consider the reasons behind students failing to graduate from college.
One of the reasons that students fail to graduate from college, in four years, is that they start off behind. They spend their first year of college taking remedial classes which do not count towards graduation. This is something that I hope to prevent (that's my job from here on out).
Another reason that students do not graduate in four years is that they are balancing a work and school schedule. I am of two minds on this. On the one hand, working during college is great, it helps reduce the debt load carried by students. On the other hand, working during college probably isn't worthwhile. In the long run, it probably costs students money.
I did this calculation a few years back. For almost all students, it does not make sense to have a summer job. They would have saved money, just by looking at the tuition spent, by taking more classes at the community college instead of working for the summer (this is assuming students move 'home' for the summer). As an upperclassman, it makes sense to take summer classes, too. The average starting salary for a college graduate is around $44,000. So, graduating a year early has the potential to be worth $44,000 (at least). It's risky, but for most students, it would probably make sense to rack up a load of student loan debt (and maybe even credit card debt, crazy as that sounds) if it means graduating a year early.
Another unexpected reason that students don't graduate "on time" is that they end up with extra credits. The average college graduate ends up with sixteen extra credits (four or five superfluous classes). This was something that was partially true for me. I graduated with an extra six credits. Another issue for students is scheduling for classes. This has been something that has been true for me as a graduate student. I would have loved to have picked up a political science endorsement, but the two classes that I needed to take were only offered once a year during the morning. So, while it won't affect my graduation, I can imagine the issues surrounding finishing off those rarely offered graduation requirements.
I'll close with a fun fact: Only 1.5% of students graduate college in three years or so. Who are these freaks?