Because the CFA is largely geared toward portfolio management - banking is transaction driven. Some of the valuation techniques carry over but beyond that I think that having the “total package” that an MBA confers carries more weight. The CFA can’t hurt you (it can really only help) - however, it will not get you the job.
In your case, with 8 years of experience, that will likely be your greatest asset. If you cannot parlay that (and all of the connections developed with banks after 8 years of working in corporate finance) into an offer or leverage your alumni connections it will be very tough to land a spot at a decent bank. It should be noted that I have very little direct experience with this topic (however, I think that I am somewhat knowledgeable). Think of it this way - you will be competing with other attornies with double-Ivy backgrounds from top-ten corporate firms that are dying to make the transition (and they have often been working first-hand with the deal teams and/or banks that they wish to join). Being from a top school or top firm buys some credibility - if you don’t have one or both of these it makes the road that much longer/tougher.