GPUs are designed with one goal in mind: process graphics really fast. Since this is the only concern they have, there have been some specialized optimizations in place that allow for certain calculations to go a LOT faster than they would in a traditional processor.
In the case of password cracking (or the molecular dynamic "folding at home" project) what has happened is that programmers have found ways of leveraging these optimized processes to do things like crunch passwords at a faster rate.
Your standard CPU has to do a lot more different calculation and processing types that what graphics processors do, so they can't be optimized in a similar manner.
