A number of reasons come to mind, the most important probably being that passing the CFA exams is not enough to become an official CFA. You also need to have significant work experience in a finance-related job, something like 3-4 years, before you can become an officially recognized CFA. http://www.cfainstitute.org/cfaprogr...m_details.html
That leads to a related topic. Obviously to accumulate the work experience necessary for the CFA, you first have to get a finance-related job. So how do you get this job in the first place? You can't just walk up to a financial services firm or a bank and just expect to get a job just "like that". Why would they hire you if you have no experience? For many people, the answer is to use the MBA to open the door to their first finance job.
That leads to yet another related topic. While it's certainly not easy to just find a job, any job, that will fulfill the work experience requirements of the CFA, that's not good enough for many of the top MBA students who are interested in finance. For them, what they want is to get a job at not just any job. They want a job with the very best firms. If you just want to work for some no-name, low-level firm, then you may not need an MBA and certainly not a premier MBA. But if you want to become an associate for Goldman Sachs or Morgan Stanley or firms at that level, then there are only a few ways to get in - a premier MBA being one of the few ways. You certainly can't simply pass the CFA exams and then just expect Goldman Sachs to call you up with a job offer. That's not how it works at all. Not even close.