The problem is largely a lack of personnel and resources. These are complex cases that require many many man-hours of investigation and analysis to prepare a prosecution. See how willing Congress is to fund that.
Congress -- especially now that the House has been taken over by the teabaggers -- does not want the regulations to exist at all. But repealing them, or even attempting to repeal them, would be a PR disaster, so Congress chooses instead not to fund their enforcement.
Different means; same result.
Reason is valuable only when it performs against the wordless physical background of the universe.
Seems like a rather lame attempt to make a point. The number of prosecutions may simply be a result of a trend to go strongly after the biggest fish rather than prosecuting as many suspects as possible.
I don't know if this is the case, but it seems to me that someone writing on the subject (or creating a graph) would want to see what is behind the numbers, rather than just publishing raw data that may be meaningless.